Dime, you overestimate the length of A-train, it is about 2/3 the screen. Painwheel is actually probably the hardest character to nab with it unless you use it like Excellebella (ie. as an anti-jump, not an anti-air)
Painwheel at fullscreen is a massive threat to BB. This is actually a matchup PW wants to and can play very patiently. BB's tools are all about forcing the opponent to make a decision and put themselves in a risky place, PW, between fullscreen flight and grounded armour NEGATES that almost entirely.
BB's jump pressure is pretty nuts, yeah, it is pretty similar to Squigly's in fact. That is why you don't want to let him get above you in this matchups, which I don't think is too much of an ask, BB struggles to attain that space unless the PW is flying at him all the time trying to pressure him.
EDIT: I want to stress how incredibly useless BB's beat extend is in this matchup, if any PW is having trouble with it, you should go into slow mo training mode and inspect the hitbox/invinicbility properties of it, it basically hits in all the places PW doesn't want to be in the first place. All of PW's effective zones are hitbox free.