If I'm not wrong pre-monthly requires wagering, and based on that will be the bonus amount. If there is no wagering, then the user will get a minimal amount fixed for the level same as that is being followed on monthly bonus. Wagering is needed to receive high bonus amount.
No, pre/post monthly bonus doesn't depend on the wager actually. We receive fixed amount during those bonuses, players recent wager doesn't create the impact (based on my experience). But weekly boost and monthly bonus amount mainly depends on the wager. From the
How do Stake's Monthly Bonuses work? page
3. How is my monthly bonus calculated?
Monthly bonuses are calculated based on your VIP level and recent wager.
Moreover, during the monthly bonus announcement Stake team has mentioned this on their telegram channel

We're too busy discussing pre-monthly didn't realize the pre-monthly for Watford bonuses wasn't out yet

We should have 2 including DRAKE x STAKE but nothing comes out.
Both of those bonuses has been distributed. We received pre-monthly in the celebration of DRAKE x STAKE partnership. And Stake made $250k (5× $50,000) bonus drop on their telegram channel in the celebration of Watford win against Southampton.