Is my fast still valid if I purposefully don’t do any prayers? And is it the same rule for both Ramadan and qada prayers?

Fasting is an obligation and praying is an obligation. Both are required. Your fasting is accepted, however, prayer is the pillar of your faith. You have to make the intention to make up the missed prayers.

During Ramadan a sin is multiplied just as a reward is multiplied in reward.