Bhishma did enumerate the avatars of Lord Vishnu and recited them to Yudhisthira after war.
He learnt from various preceptors who were renowned and even Narada and Vyasa told that only Bhishma can instruct Yudhisthira burning in grief on the account of his own kin.
'Then Vyasa, O monarch, that foremost of all persons conversant with the Vedas, casting his eyes upon that ancient and all-knowing person, viz., Narada, said, If, O king, thou wishest to hear of duties and morality in full, then ask Bhishma, Conversant with all duties and possessed of universal knowledge, that son of Bhagirathi will remove all the doubts in thy heart on the difficult subject of duties. That goddess, viz., the genius of the celestial river of three courses brought him forth. He saw with his physical eyes all the gods with Indra at their head. Having gratified with his dutiful services the celestial Rishis having Vrihaspati at their head, he acquired a knowledge of the duties of kings. That foremost one among the Kurus obtained a knowledge also of that science, with its interpretations, with Usanas and that regenerate one who is the preceptor of the celestials know. Having practised rigid vows, that mighty-armed one acquired a knowledge of all the Vedas and their branches, from Vasishtha and from Chyavana of Bhrigu's race. In olden days he studied under the eldest-born son of the Grandsire himself, viz., Sanatkumara of blazing splendour, well conversant with the truths of mental and spiritual science. He learnt the duties in full of the Yatis from the lips of Markandeya. The bull among men obtained all weapons from Rama and Sakra. Although born among human beings, his death itself is still under his own control. Although childless, yet he has many regions of bliss hereafter as heard by us. Regenerate Rishis of great merit were always his courtiers. There is nothing among objects that should be known that is unknown to him. Conversant with all duties and acquainted with all the subtle truths of morality, even he will discourse to thee upon duty and morality. "
So Bhishma was not just a skilled warrior but was good at Vedas also.
"Learning by his Yoga prowess of the devotion of Bhishma, Madhava, otherwise called Hari, (entering his body) bestowed upon him heavenly knowledge compassing the Past, the Present, and the Future, and went away. "
Later, Bhishma told Yudhisthira various types of stories of olden days which his preceptors taught to him and also tells what Vishnu told to Narada when he showed his celestial form to him and Vishnu tells to Narada the conversation he had with Brahma and what he told him the beginning of the Kalpa to Brahma because of the boon that Krishna gave to him which is to know the heavenly knowledge of the past, present, and the future.
these are the avatars of Varaha, Narasimha, and Vamana avatars.
I shall, assuming the form of boar and putting forth my strength, bring back the Earth with her belt of seas to her own position for the good of all creatures when she becomes submerged in water. I shall then slay the son of Diti, named Hiranyaksha, filled with pride of strength. Assuming the form then of a Man-lion (Narsingha), I shall, for benefiting the deities, slay Hiranyakasipu the son of Diti, who will be a great destroyer of sacrifices. Unto Virochana (the son of Prahlada) will be born a mighty son of the name of Vali. That great Asura will be unslayable in the whole universe consisting of deities, Asuras and Rakshasas. He will hurl Sakra from the sovereignty of the universe. When after routing the Lord of Sachi, that Asura will take unto himself the sovereignty of the three worlds, I shall take birth in Aditi's womb, by Kasyapa, as the twelfth Aditya. I shall (taking the sovereignty of the three worlds Vali) restore it to Indra of immeasurable splendour, and replace the deities, O Narada, in their respective stations. As regards Vali, that foremost of Danavas, who is to be unslayable by all the deities, I shall cause him to dwell in the nether regions.
Bhishma later describes and tells how Vishnu told Narada how he would take birth as Parashurama and also as Rama, son of Dasaratha during the end of Treta Yuga.
In the Treta age I shall take birth as Rama in the race of Bhrigu, and exterminate the Kshatriyas who will become proud of their strength and possessions. Towards the close of Treta and the beginning of Dwapara, I shall take birth as Rama, the son of Dasaratha in Iskshaku's royal line. At that time, the two Rishis viz., the two sons of Prajapati, called by the names of Ekata and Dwita, will in consequence of the injury done by them unto their brother Trita, have to take birth as apes, losing the beauty of the human form. Those apes that shall take birth in the race of Ekata and Dwita, shall become endued with great strength and mighty energy and will equal Sakra himself in prowess. All those apes, O regenerate one, will become my allies for accomplishing the business of the deities. I shall then slay the terrible lord of the Rakshasas, that wretch of Pulastya's race, viz., the fierce Ravana, that throne of all the worlds, together with all his children and followers.
Here Vishnu describes how he will be born as Krishna and tells what he will do when he is born as Krishna
Towards the close of the Dwapara and beginning of the Kali ages, I shall again appear in the world taking birth in the city of Mathura for the purpose of slaying Kansa. There, after slaying innumerable Danavas that will be thorns in the side of the deities, I shall take up my residence in Kusasthali at the city of Dwaraka. While residing in that city I shall slay the Asura Naraka, the son of 'the Earth,--him, that is, who will do an injury to Aditi, as also some 'other Danavas of the names of Muru and Pitha. Slaying also another foremost of Danavas, viz., the lord of Pragjyotisha, I shall transplant his delightful city furnished with diverse kinds of wealth into Dwaraka. I shall then subjugate the two gods worshipped of all the deities, viz., Maheshwara and Mahasena, who will become fond of the Danava Vana and do him diverse good offices and who will exert themselves vigorously for that worshipper of theirs. Vanquishing next the son of the Danava Vali, viz., Vana, who will be endued with a thousand arms, I shall next destroy all the inhabitants of the Danava city called Saubha. 1 I shall next, O foremost of Brahmanas, compass the death of Kalayavana, a Danava who will be endued with great might in consequence of his being equipt with the energy of Gargya. A proud Asura will appear as a king at Girivraja, of the name of Jarasandha, who will quarrel with all the other kings of the world. His death will be compassed by me through someone else guided by my intelligence. I shall next slay Sisupala in the sacrifice of king Yudhishthira, the son of Dharma, which sacrifice all the kings of the world will bring tribute. In some of these feats, only Arjuna, the son of Vasava, will become my assistant. I shall establish Yudhishthira with all his brothers in his ancestral kingdom. People will call me and Arjuna as Narayana and Nara, when, endued with puissance, we two, exerting our strength, shall consume a large number of Kshatriyas, for doing good to the world. Having lightened the burthen of the Earth according to our pleasure, I shall absorb all the principal Sattwatas as also Dwaraka, my favourite city, into my own self, recollecting my all-embracing Knowledge.
Here, Vishnu lists his avatars to Brahma.
Endued with four forms, I shall, in this way, achieve many feats of great prowess, and attain at last to those regions of felicity created by me and honoured by all the Brahmanas. Appearing in the forms of a swan, a tortoise, a fish, O foremost of regenerate ones, I shall then display myself as a boar, then as a Man-lion (Nrisingha), then as a dwarf, then as Rama of Bhrigu's race, then as Rama, the son of Dasaratha, then as Krishna the scion of the Sattwata race, and lastly as Kalki.
So yes, Bhishma recited the Vishnu Sahasranama and listed the avatars of Vishnu, praised Krishna in the presence of Krishna and instructed Yudhisthira how to be a capable emperor by teaching him mysteries of moralities, the avatars of Lord Narayana and many other olden stories but the only thing is he did not list them in Sabha Parva. He only says that Lord Krishna is the lord and master of the universe and tells that according to him, Krishna righteously deserves to be the guest of Yudhisthira's Rajasuya Yagna and even tolerates when Shishupala insults him. Bhishma also tells Bhima not to be angry at him by telling birth secret of Shishupala as to how only Krishna can slay him but Bhishma recites the avatars of Lord Vishnu in Shanti Parva, SECTION CCCXL which is Section 340.