For email migration / Proton:

• Proton has an import tool but I think it's still for paid customers only. You could pay for a 1 month subscription and do your thing.
• The import tool has labels matching, pay attention to them if you want to migrate them too. If you have multiple addresses, create a label for each when importing so you'll have an easy time identifying/filtering later.
• I would encourage a paid Proton subscription if you can afford it, for its extra features. I.e. unlimited folders / labels; I also use the Export tool from time to time to backup all Proton email messages offline.
• When changing your email address for accounts, you need to make sure all are accounted for in your password manager, and use a status for each of them (ie To migrate, Migrated, Cannot migrate / To delete, Cannot migrate / create new). This is going to be a tedious process, but it will be rewarding at the end.
• Don't leave your accounts that you cannot migrate in the air (ie they don't have a change email option), even if they're not important. Delete them. You might have to contact support on some of them to try to change email or request deletion. Consider this a spring cleaning and make the efort.
• When deleting accounts, use the GDPR option if possible / you're in the EU.
• Keep your old Gmail around for some time to catch any accounts linked to it that you might have missed. As somebody else mentioned, there might be some for which you used Gmail login and those are easy to miss, especially if they don't send any emails. You won't be able to recover them without access to your Google account.
• I wouldn't bother with forwarding emails (why let Google know of your new identity?). Delete emails already imported. Use the import tool multiple times to import any new ones.
• When you do the email address migration, even if you didn't have multiple email addresses in Gmail, this could be a good time to separate online identities and have multiple addresses and/or aliases in Proton (ie 1. for personal/official/utilities accounts - your real identity, 2. shopping - still real identity, but these might be spammier, 3. rest/disposable/not tied to your real info/no payment enrolled; even more, depending on your use case). Any Proton paid plan allows you to have multiple addresses under the same login (10 for Mail Plus for example).
• Personal opinion: Proton is awesome. Every year, even on the cheapest Mail Plus plan, Proton awards a 1GB Storage Bonus to all paying users.
• The free plan has a limit of only 1 custom filter (they used to limit them for Mail Plus too, some time ago). To bypass that (Proton even encourages it because it's more efficient for their servers), learn Sieve Filters, and that way you can group multiple filters into one sieve (or have all of them in one sieve, if on the free plan). You can use comments in sieve filters.
• Proton supports the "+" in the address, just like Gmail does. It's a quick way of creating aliases.
• The Proton password manager also has some feature of creating aliases (for paid plans) - they call them "hide-my-email aliases"; but it's limited for the lower tier plans (10 for Mail Plus), and maybe you wouldn't want to bother with it since you won't be using it as your actual password manager.
• Something I learned the hard way, don't use a short 3-4 characters username / email address (probably hard to find any available anymore, as Proton exists for some time); it will attract more spam from spammers randomly generating email addresses / generating them from a dictionary.

For Youtube, on Android:

• I use Youtube in Brave browser, which for the time being can still block the ads, and also keeps playing in the background.
• When I want to avoid Youtube, for Youtube links, I use UntrackMe (F-Droid), which (among others) redirects to an Invidious instance opened in the browser. Initially I installed it for Twitter, but Nitter doesn't work anymore.

Cloud storage:

• I'd go the self hosted route - NextCloud + DAVx5 (contacts sync), and VPN to access it when out of home (if needed; otherwise, set it to sync over unmetered WiFi connections only, and mark your home WiFi as unmetered). But this is me - I could probably safely use Proton Drive, but wouldn't have the same flexibility and would force me to a higher cost plan. For you, this means entering homelab territory and it gets complicated. But IF you do, there are other self-hosting apps you could benefit from (ie PiHole, Jellyfin, Home Assistant if you're into home automation, etc).

2FA app:

• Never tried Authy, but I use Aegis and it's good. Open source, it has backup, export, custom icons for entries, (bulk) import via QR, etc.

Video player:

• You could try VLC if you need subtitles support.