Tried same setup as you today and got same 15mm sag with 548 spring, but interestley bike rode fine with that little sag.1 turn of preload on my spring, just to eliminate free play.
And yes, long link needs way lower spring rate.
I would say 25% is pretty small SAG, not much small bump sensitivity.
Did you have your TTX22m.2 changed to 230x65 or you are getting a new shock?
Have you tried it with 19mm sag?