US-Michigan—Subpanel install—Can I use PVC conduit and THHN in-wall to go from disconnecting means to panel inside the garage?
Sch 40 is fine, this time at least

Since this conduit run is inside a wall, its generally not subject to physical damage, which means Schedule 40 PVC is fine for this job.  I'd simply bring it out of the back of the box with a LB body, by the way, and then it can just pipe straight into the side of the indoor subpanel, or into the bottom or top with a LL or LR body if that is more convenient.

As to other means? PVC is really your best choice here, unless you want to go metallic instead, which puts you in EMT-land.  (Rigid is a bigger pain to deal with due to threads, and SE cable is going to be harder to replace later if you want more power in the garage.)  I'd just size it generously -- 2.5" is a good size for this.

Wire derating is not needed here

Insulation isn't an issue when it comes to derating for wire in conduit as long as the ambient temperature isn't roastingly hot (i.e. not greater than 30&deg;C/86&deg;F).

Don't forget the grounds

Since you're providing a feeder to a detached outbuilding, you'll need to both run an equipment grounding conductor back to the source of the feeder (it can be 6AWG copper) to provide a fault path, and run some more 6AWG copper back to a grounding electrode (ground rod) for the outbuilding to deal with lightning-type problems.  Don't forget to pull the bonding screw in the disconnect while you're at it!