I'm sorry I left this unanswered for so long! I missed the original notification and then spent like a week just thinking about this like "do I even have English sources? How embarrassing!" I mean, I'm far from an expert, tbh. I just spent four years reading way too much about Peter III lol. No, but ok... I think I've referenced most of the accessible English material on Peter in my yuletide/exchange letters and other posts, other than maybe a few niche articles (unless something new has come out in the last 2-3 years while I've been less obsessed). So if you've read those, you probably have the references. (Though, I'll say, if you don't want to read Leonard's dense af book on politics and policy, Marc Raeff's article "The Domestic Policies of Peter III and His Overthrow" is quite good and I don't think I've mentioned that oen before.)

I can recommend a couple of things on Paul I, though! First, Roderick McGrew has a really good biography. (I happen to have a scan of my copy if there's interest >_>) Another is Paul I: A Reassessment of His Life and Reign edited by Hugh Ragsdale with multiple contributors.

If you'd like to do some digging around in primary sources, the Vorontsov archive is available online: https://runivers.ru/lib/book8295/471943/
Unfortunately, I don't know how accessible it is if you don't know Russian, but there are many letters in French if that's a language you're comfortable with.

Agh I feel like I could be more helpful, but every time I think of something to recommend I realize it's in Russian lol. (Sometimes, if it's primary-source based, it's in freaking horrible old print font + old(er) Russian with weird ass syntax so it's not even like you could reasonably google translate that.) But, frankly, if you're looking for something specific I'm sure I could look around and let you know?

ETA: Oh God, Massie the Catherine stan...my nemesis...