you might be surprised at how much variation there is between all your bikes
toptube length in bmx is very important ( well , from the mid 90s it has been )
the only bike i can ride under 20.5" is my dirtbike at 20.25" . i own that purely because i couldnt afford it in 94 . even 20.5" is a tad short . 20.75" is ok if i angle the bars back slightly 21" is too long unless i wanted to ride very quickly on a track , but i could just angle the bars forwards slightly with a 20.75" . i have one frame thats 20.6"
and you can genuinely feel that little extra over a more common 20.5" . then theres chainstay length . over 14.2" and i cant hop it . under 13.6 and i struggle to keep the front wheel on the ground . there was a point where i was riding often enough that i could feel a frame with a difference in head angle of .5 of a degree
"It also has a seat. I don’t use it, but you probably will for the first few years until you build up the muscle."