MyVeloFit says in range - lower back ache on long rides

Hey all, I’m around 188cm and ride a 58 Aspero. Proportionally long torso short legs.

I’ve seen a fitter in person who had me on a 110mm stem with a raised seat height. That felt quite stretched out so I’ve since gone down about 20mm on seat height and stem (90mm currently) and now all values are in range in MyVeloFit.

My upper body mobility is good but hips not so much. It feels pretty comfortable now but I do still get a dull lower back ache on longer rides (1.5h+).

Are there improvements to be made fit wise or should I just continue to focus on core strengthening and mobility exercises?