I have a number of characters that I'm playing - swapping the save file sets into per-character folders - but I'm having a problem trying to remember what main quests they've done. Only an active quest shows in the log, so that doesn't help between them.

Using F9 and ";" to see the quest script variables for the currently running quests helps, but takes a lot of decoding, cross-checking with the decompiled "source". Is there an easier way?

Sadly, I think the only other way is to take notes. Daggerfall, like Skyrim, has a pretty mediocre journal. It was common in old games to expect players to take notes, though. I don't know about Daggerfall, but lots of old game manuals even had pages included for it.