Just to update, things I have tried (or in some cases tried again) since I have posted this:
Downgrading drivers to 16.11.5, doing a clean install using DDU: Went about 4 hours of gaming crash-free, then started acting the same as before.
Using Clockblocker: No apparent change.
Set Radeon Settings to override application anti-aliasing (suggested by someone on Reddit): No change.
Re-upgraded to the latest drivers with another clean install: No change.
Removed Radeon Settings (and hopefully Wattman) via the Change option in Programs &Settings: Didn't crash yet for a few hours last night, will wait and see.