Now I only register single-use throwaways on Protonmail (despite their efforts to detect and stop it).

That kinda sux. There are plenty of other, more suitable, services for that. I would recommend not wasting this great service for such purposes.

I don’t know if that makes much sense. If you lose, for example, assuming you spread it over three providers, 1/3 of your accounts, wouldn’t this already be bad enough? I think, the overhead of using so many different services simultaneously is way bigger and more real, than something bad happening to the single service I settled with. Am also with Proton for many many years (back then, it was only “ProtonMail”…) and nothing bad happened. It only got better & better over the years. It’s amazing.

Switching to Proton from all the other shit accounts was one of the best online services thing I ever did in my life. I got a discount back then and to be honest, I would even pay double the price, if I had to. It’s just worth it.

As to what you need to know… There is not much to know, except, just do it. Do not hang onto the obsolete accounts. Migrate everything to Proton, then keep the old accounts for 6-12 months, just to make absolutely sure, you did not miss some rare account you barely ever use and is still connected to the old e-mail address. Finally, just never log into the old one ever again and stay with Proton. Proton is king.

If I remember correctly, Proton even offers migrations features, which let you migrate from Google to Proton in some mouse clicks.