looking to replace those thin shoulder straps with something that doesn't cut into the shoulder. any suggestions?

Get some of those .50 cent rats off the beav...and sew them onto the straps

I mounted one to a very light aluminum pack frame that had good straps and a damaged pack bag. Much better!

junk auto seat belts just

Padded backpack straps can be attached rather easily.
I have WW 2 web belt suspenders on mine. But I made my basket extra strong so I can sit on it without fear of cracking it.

Amazon....type in Smith Performance Sprayer padded nylon straps

I mounted mine onto one of those Coleman plastic pack frames with good padded shoulder straps but more importantly, a padded hip belt to distribute the weight. The key if you mount it to a frame is to make sure the pack is as close to your body as possible. If it hangs out there a few extra inches, you’ll feel it.

I bought some from one of the outdoor vendors at a trapping rendezvous. I think it was military surplus

Old fire hose?