I try to get all my bikes to the same measurements, probably failing miserably compared to many members.

BUT, being a bit different is for me - THE POINT.
If they all fit feel and ride the same, why have more than 1?

i confess, most of my rides are sub 40 miles.
For longer rides I have a few favorites; the choice depends on terrain.

Good luck finding a solution to the back pain. Ride it more - you’ll find a solution or dump it. Is it a Trek?

edit: does the painful one have a shorter head tube?