For whatever reason- despite having it configured to be "On", I've had to manually turn it on, aka go into the UI and click on the domain name to check it, for it to work.

Sounds like a minor issue. But I'll often not realize that I'm in light mode on a site especially when I'm getting tired. I wont realize that my eyes are straining, and when I do, I think: Monitor brightness is low, so what could it be?
...Ohhh, right. Dark Reader isn't doing anything again. Sigh

Anyways, is it really true that this addon is responsible for memory leaks? extensions are sometimes hounded by baseless accusations of malware and stuff, but its happening more and more often that extensions actually are the culprit behind browser performance issues