One thing I really miss with flash drives, a write protect switch.
Yes, I can mount 'ro', but many (all?) Linux Desktops mount the drive write through its GUI. I have aliases for mounting and never used the pointy-clicky mount in DEs.
FWIW, I am usually in fvwm or cwm, depending on screen size, so I mount 'ro' when I want to. But a hw switch on flash drives would have been nice :)
With the default ext4 filesystem, mounting 'ro' doesn't actually prevent writes to the disk. For that you need 'noload' too, to prevent journal replay. This can result in a broken filesystem if it wasn't unmounted cleanly, although assuming it's 100% read-only it shouldn't do any permanent damage (perhaps enforce read-only access at the block device level to be sure).
You might like: https://www.kanguru.com/products/kanguru-ss3
Using an SD card (or micro SD in an adapter) connected to a USB reader might meet your needs. You can then use the SD write protect switch.
I thing I learned only recently is that the write protect switch on the SD card is not an electrical switch connected to anything in the SD card itself: it just hits a lever in the SD socket that opens a contact closure and it's up to the system (hardware and software both) to bother to look at it. So on many systems the write protect switch doesn't even work.