Okay, slight update. Rebooted, commit charge went down from over 800 MB to around 450, with >400 MB of RAM used. REPAIR APP did nothing. Rebooted again, down to ~300 MB of RAM used, commit charge still ~450 MB. Started Brave (no page loaded, just the browser) - Avast panics, hogs hard drive and both cores of CPU for perhaps 30 seconds, RAM usage up to ~500 MB temporarily. This is straight off a fresh reboot.
I guess it's time for another avastclear. What happened to taking less than 30 MB of RAM, with less than 100 MB of commit charge?
Once I've fixed this, I'll give it a bit of uptime and see if I can reproduce the catastrophic high-memory event I got last night.