When we finish getting all bad returns and keep doing only good karma, then we would keep
getting only good returns. Is that so? Then when would we get moksha?
Yes, what you sow is what you reap. But if you will do Nishkama Karma
(actions without attachment), you will not be bound by this law.
In Shrimad Bhagavad Gita 3.9 lord Krishna says:
यज्ञार्थात्कर्मणोऽन्यत्र लोकोऽयं कर्मबन्धनः।
तदर्थं कर्म कौन्तेय मुक्तसंगः समाचर।।3.9।।
English Translation By Swami Gambirananda
3.9 This man becomes bound by actions other than that action meant for God. Without being attached, O son of Kunti, you perform actions for Him.
English Translation Of Sri Shankaracharya's Sanskrit Commentary By Swami Gambirananda
3.9 Ayam, this; lokah, man, the one who is eligible for action; karma-bandhanah, becomes bound by actions- the person who has karma as his bondage (bandhana) is karma-bandhanah-; anyatra, other than; that karmanah, action; yajnarthat, meant for Got not by that meant for God. According to the Vedic text, 'Sacrifice is verily Visnu' (Tai. Sam. 1.7.4), yajnah means God; whatever is done for Him is yajnartham. Therefore, mukta-sangah, without being attached, being free from attachment to the results of actions; O son of Kunti, samacara, you perform; karma, actions; tadartham, for Him, for God. An eligible person should engage in work for the following reason also:
English Translation of Ramanuja's Sanskrit Commentary By Swami Adidevananda
3.9 The world is imprisoned by the bond of work only when work is done for personal ends, but not when work is performed or money acired for the purpose of sacrifice etc. prescribed in the scriptures. So, for the purpose of sacrifice, you must perform acts like the acisition of money. In doing so, overcome attachments generated by the pursuit of personal ambitions, and then do your work in the spirit of Yajna. When a person free from attachment does the work for the sake of sacrifices etc., the Supreme Person, propitiated by sacrifices etc., grants him the calm vision of the self after destroying the subtle impressions of his Karmas, which have continued from time without beginning. Sri Krsna stresses the need for sustenance of the body solely by the remnants of sacrifices in respect of those who are devoted to all ends of human life. He decries the sin of those who nourish the body by things other than the remnants of sacrifices:
English Translation of Abhinavgupta's Sanskrit Commentary By Dr. S. Sankaranarayan
3.9 Yajnarthat etc. Binding are the actions which are different from the one that is Yajnartha, i.e., the one that is to be performed necessarily. The action, that is to be performed necessarily, does not yield any fruit, if it is performed with no attachment for the fruit.
Again in Shrimad Bhagavad Gita, 3.19 lord Krishna says:
तस्मादसक्तः सततं कार्यं कर्म समाचर।
असक्तो ह्याचरन्कर्म परमाप्नोति पूरुषः।।3.19।।
English Translation By Swami Gambirananda
3.19 Therefore, remaining unattached, always perform the obligatory duty, for, by performing (one's) duty without attachment, a person attains the Highest.
English Translation Of Sri Shankaracharya's Sanskrit
Commentary By Swami Gambirananda
3.19 Since this is so, therefore, asaktah, remaining unattached; samacara, perform; satatam, always; karyam, the obligatory; daily karma, duty; hi, for; acaran, by performing; (one's) karma, duty; asaktah, without attachment, by doing work as a dedication to God; purusah, a person; apnoti, attains; param, the Highest, Liberation, through the purification of the mind. This is meaning. And (you should perform your duty) for the following reason also:
English Translation of Ramanuja's Sanskrit Commentary By Swami Adidevananda
3.19 Therefore, considering that work has to be performed with detachment, you perform it, considering yourself a non-agent. This will be declared in the words 'with detachment' and 'which ought to be done,' meaning that one attains the Supreme by Karma Yoga itself.
English Translation of Abhinavgupta's Sanskrit Commentary By Dr. S. Sankaranarayan
3.17-19 Yas ca etc. upto purusah. However, for a person who rejoices in the Self and performs action simply as a [routine] business of organs of action, there is no difference between (his) action and nonaciton. That is why he inflicts punishment on, or does favour to, every being, not with desire for any gain for himself, but with a conviction that it a is thing that deserves to be performed. Therefore, just unattached, one should perform action that is to be performed.
And by doing Nishkama Karma
you will be freed from the bondage of birth. It is said by Lord Krishna in Shrimad Bhagavad Gita 2.51.
कर्मजं बुद्धियुक्ता हि फलं त्यक्त्वा मनीषिणः।
जन्मबन्धविनिर्मुक्ताः पदं गच्छन्त्यनामयम्।।2.51।।
English Translation By Swami Gambirananda
2.51 Because, those who are devoted to wisdom, (they) becoming men of Enlightenment by giving up the fruits produced by actions, reach the state beyond evils by having become freed from the bondage of birth.
English Translation Of Sri Shankaracharya's Sanskrit Commentary By Swami Gambirananda
2.51 The words 'phalam tyaktva, by giving up the fruits' are connected with the remote word 'karmajam, produced by actions'. Hi, because; [Because, when actions are performed with an attitude of eanimity, it leads to becoming freed from sin etc. Therefore, by stages, it becomes the cause of Liberation as well.] buddhi-yuktah, those who are devoted to wisdom, who are imbued with the wisdom of eanimity; (they) becoming manisinah, men of Enlightenment; tyaktva, by giving up; phalam, the fruit, the acisition of desirable and undesriable bodies; [Desirable: the bodies of gods and others; undesirable: the bodies of animals etc.] karmajam, produced by actions; gacchanti, reach; padam, the state, the supreme state of Visnu, called Liberation; anamayam, beyond evils, i.e. beyond all evils; by having become janma-bandha-vinirmuktah, freed from the bondage of birth birth (janma) itself is a bondage (bandha); becoming freed from that , even while living. Or: Since it (buddhi) has been mentioned as the direct cause of the elimination of righteousness and unrighteousness, and so on, therefore what has been presented (in the three verses) beginning with, 'O Dhananjaya,৷৷.to the yoga of wisdom' (49), is enlightenment itself, which consists in the realization of the supreme Goal, which is comparable to a flood all around, and which arises from the purification of the mind as a result of Karma-yoga. [In the first portion of the Commentary buddhi has been taken to mean samattva buddhi (wisdom of eanimity); the alternative meaning of buddhi has been taken as 'enlightenment'. So, action is to be performed by taking the help of the 'wisdom about the supreme Reality' which has been chosen as one's Goal.]
English Translation of Ramanuja's Sanskrit Commentary By Swami Adidevananda
2.51 Those who possess this evenness of mind while performing actions and relinish their fruits, are freed from the bondage of birth, and go to the region beyond all ills. 'Hi' means that this dictum or teaching is well known in all the Upanisads.
English Translation of Abhinavgupta's Sanskrit Commentary By Dr. S. Sankaranarayan
2.51 Karmajam etc. The persons who are endowed with the determining faculty with regard to the Yoga, renounce the birth-bondage, by renouncing the fruit of actions; and they attain the Brahman-existence.
And now come to your second question
One would say that karma ends when we get moksha. If that's so, then what happens to our
Sanchita Karma?
So, if we will renounce the fruit of actions, then there will not be any Sanchita Karma (It's my understanding).
Please note that this answer is only based on BG.