We can set up a dwarf with any strength we want (and thus any maximum jump distance) using an many Manuals of Gainful Exercise as needed, and ther are plenty of speed reducers that cut your speed in half but do not state they reduce your maximum jump distance. We can get a PC down to a speed of 5 feet and a jump distance of 1000000000000000000000 if we want to really push it.
The most common situation we'll see, however, that demonstrates the problem at the core of this thread is a PC that dashes and wishes to launch themselves off the ground right before that second set of movement ends. You can fudge a bit to have them be right before launch or right after lnding rather than mid-flight if you want, but at some point the fudging is stretched too far and you need a way to handle it... and thus we have the suggestions above.
If a PC jumps and will spend more than 6 seconds in the air before landing, I see no reason to pretend they're being forced to do physical feats to stay up there... they're supported mostly by magic. Thus, I would allow the PCs to do something while in the air with their action.