I think a lot about what it means to that we don't often KNOW what we are doing with our bodies and even when we DO know what we are doing, we don't know WHY.
We've are naturally indoctrinated by our culture (look thinner, dammit!) and we are given watered-down fitness/exercise/movement advice (naval-to-spine always) and we end up forgetting to think critically about what we're doing or haven't been taught how to think critically.
I know this is hard with the belly. We have SO many feelings about our bellies and what happens, or doesn't happen, inside them.
Does this resonate?
Are you willing to explore changing your mindset about what your belly should look like and should be doing in order to allow a more functional, healthy process to exist?
No matter what, I encourage you to play around with NOT sucking in (really - can you let the upper belly go? the lower? the right? and the left?) and seeing how the core responds to movement without you consciously deciding how much effort is required.
If you want true, reflexive, functional core strength, that's your place to start.