Just to follow up… I decided to go with a 0.020" thick PEI with pre-laminated adhesive available on Amazon.
The install was quite a bit harder than with the stock 0.010" sheet. First, because the 0.020 sheet is stiffer it is more difficult to apply and get all the bubbles out as you gradually lay down the sheet onto the glass. Second, trimming the excess was much more difficult: Whereas I’ve always been able to easily trim excess 0.010" sheet with just a hand-held razor blade, the 0.020" sheet is much harder to cut. I resorted to a utility knife, finally got it all trimmed, but it wasn’t fun.
When I peeled off the top protective sheet, I found the PEI was perfectly smooth and shiny – not matte textured like the sheets from Lulzbot. Grabbed some Scotch-brite gray (equivalent to 00 steel wool I think) and textured it myself.
Ran a test print and everything worked perfectly. No need for Z offset changes as washers are still on top of the sheet, and no need for shims or longer spacers under the washers, as the extra 0.010" is easily accommodated by the “cushion” of the ninjaflex bed corners.
Only time will tell if the .020" holds up better than the stock .010" sheet. It is my impression that a lot of the bubbles occur due to the sheet “stretching” during part shrinkage, so the thicker sheet should provide more resistance to that. I’ll try to report back here in a few months with results.