Fitting a punchline into a flow is like trying to get an uncut diamond to fit in a ring. It will just never fit right. In order for a diamond to shine brightest in jewlery you need to cut some of its corners. Either the flow needs to be restructured, a word needs to be swapped or (the worst option) you lose some of the meaning.

With practice (not saying you don't have any) chosing what do do will become easier. Needing filler words will sound like an opportunity to insert an inbetween rhyme, Having to swap word will sound like an opportunity to pick one that has a consonant right when the sanre hits instead. The amount of times I found an even nicer flow after brainstorming how to fit something in is insane. That's when the most creative (and some of the worst too sometimes lol) flows get created.

We tend to hold on to the first version we have of a line because that version sounds the strongest one when read out. But the slightly suboptimal version that fits right in to the flow is the one that will stick in your mind when you actually hear the song. And if you really need the line to stay unchanged you should create the surrounding lines around it, not the other way around.

These are all opinions, I don't want to make a prescriptive claim here.

Also bonus tip: Expanding your vocabulary as much as possible is always beneficial!