I guess they only had to follow Grihastha ashrama when they attained the appropriate age, learning and renunciation being denied to them.
Did Sudras have Varnashrama Dharma as opposed to Varna Dharma only?
Yes, Shudras are considered lifelong Grihasthas.
Shudras only belong in one Ashrama as per Gautama Smriti:
- Live with his wife (only)
'Another commentator explains the Sûtra to mean that he shall live with his wife only, and never enter another order (i.e. never become a student, hermit, or ascetic).'--Haradatta.
Some more verses:
Yogi-Yājñavalkya (Parāśaramādhava, p. 537).—‘There are four stages prescribed by the Veda for the Brāhmaṇa, three for the Kṣatriya, two for the Vaiśya and one for the Śūdra.’
Vāmanapurāṇa (Do.).—‘Four stages have been spoken of for the Brāhmaṇa; three only, i.e., those of the Householder, the Student, the Hermit, for the Kṣatriva; only two—Householdership and Hermitship—for the Vaiśya; only one, that of Householdership, is proper for the Śūdra.’
Smṛtyantara (Aparārka, p. 966; Parāśaramādhava, p. 538).—‘Having paid off the three debts, being freed from all notions of I and mine, the Brāhmaṇa, the Kṣatriya and the Vaiśya, [and not the Shudra] may go forth from the house.’