I agree with @Ordog. Sounds like a very niche feature that not many people would use, and it would just add too much complexity.
If you only want to Flex some of a payment, just Flex it, and then us some of the money Monzo gives you back into your account to pay off the amount you didn’t want to Flex.
Or
Pay for the things you don’t want to Flex, then pay for the stuff you do want to Flex seperately, ideally on the Flex card directly. (Most places will let you do this)
Let’s say my credit limit of Flex is £1,000. I make a purchase of £2,000, and would like to flex the £1,000 that is available. Currently it’s not possible, I’d have to pay multiple times so that the transaction would be less than my Flex available money.