Why not just get a second hand MTB with a triple crankset already on it?
Then just buy a rigid fork.
My main training steed still has a triple 3x9. Until I get fitter and start riding road bikes again (compact double).
I just find the gearing easier and more closely spaced. Got a fairly tight...
I ran into the same problem with seatpost angle.
Angle is fine for true downhill applications (steep terrain).
But on more tame shallower runs saddle will be nose in the air.
I eventually got a FSA SLK seatpost and put these z-race style lightweight yokes onto it...
There are some on aliexpress:
https://www.aliexpress.com/item/1005002467366726.html
https://www.aliexpress.com/item/1005005192895759.html
They may be fake, I don't know. Possibly. Probably.
I got some anyway.