> Then the week before fixit, each subteam goes through these bugs and sizes them:

I advocate to never size/score bugs. Instead, if your process demands scores, call everything a 2 because over the course of all the bugs, that will be your average. You'll knock out 10 small ones and then get stuck on a big one. Bug-fixing efforts should be more Kanban than Scrum. Prioritize the most important/damaging/whatever ones, do them in order, and keep doing them until they are done or you run out of time.