Like this. It could be even cooler if you could just put the corners wherever you wanted, so you could make the lines wrap around other quests instead of going through them like this. Apologies for the bad MS Paint drawings.

Unrelated, but is there a way for Retrieval quests to complete without having to click the detect button? Right now I have to open the quest, hit the detect button, exit the GUI, then reopen the quest for the quest to update as completed.

Custom line renders are already a thing as part of themes but the old BQ2 versions aren't the easiest to write them for:

They can even be animated if someone's willing to put the work in:

Retrieval quests should auto-complete just fine (as long as they aren't set to consume). Just be sure you're not in edit mode before testing that. If it's still not auto-completing let me know.