Following up on this, emailing the company led to finding out that the shelf
panels are 0.068", and the other panels are only 0.045".

Is it just me, or is something that thin likely to be pretty flimsy/cheesy?

Those dimensions are in the range of thickness of vinyl siding, to
give you a benchmark for judging flexibility. You are not likely to
find thicker material in similar products because a common
manufacturing process is vacuum forming and thicker material means
much slower process cycles.
FWIW, in our local box stores there are some Sterling products worth
looking at as Derby Dad suggested. Sterling is a Kohler product with
top notch customer service.
Regarding your shy of 60" dimension, don't let that push you into a
shabby compromise. Take down the wall board, whatever, and modify it
to gain the width you need, trimming oversized studs as needed. A
clever remodeling contractor could do this neatly at a fair price. DIY
is probably possible, but we can't judge that off site.
You probably realize by now that your budget will have to be
Obamaized. Start with the basics, a good substrate, a decent product,
a careful install and you'll still be pleased a decade hence.

Joe