the snap-based chromium cannot access files on my separately-mounted /opt filesystem. The non-snap chromium has no such limitation. Until or unless the snap version ever is able to access all the filesystems on my device, I am willing to live with the risk of a PPA-based version.

Here is a link to install a deb version of chromium, seems like it be easier to use another browser myself.

Not sure but might be a chromium snap problem. Snaps have very few permissions, can try going to software centre/store and see if you can give more permissions, should just be on/off switch, or might need to use another browser(deb not snap). Chromium might have a deb only version now again, but not sure if for 19.10 or only 20.04.

The strangest "quirk" I had was that I couldn't get the web browser to save a file directly to an attached, encrypted drive. Permissions problem. So I had to save to an interim folder then move it across by hand. Utter pain.