The incremental approach is always the better solution imo. For email, for password managers, for any big migration. People always put off saying it'd be a lot of work, but if you start at any given time, within 1-2 year you'll be fully migrated.

This is what I did for password manager, I migrated each website on a per-use basis. Every time I used a website with a manual password, I replaced it with a generated password. This automatically prioritizes more important websites, which you tend to use more often.

For email, similarly, you migrate to the new email, and you give people the new one gradually too. Eventually anything that's left over will be spam, and that'll actually be your "less-important" secondary address you can give to websites you don't care about.

What is Google's policy on accounts set up to forward all mail re: scanning contents of forwarded messages?

I don't think it really matters; Google has a track record of abusing privacy (regardless of what the policy is.)

s/policy/practice/

Published "privacy policies" seem to be a way of preserving opacity. It is nigh impossible, absent litigation discovery, to decipher what the company is actually doing.