Mail configuration possibilities

I have 3 email accounts. Two are with Hotmail/Outlook and one is with Gmail (but managed by a different agency). When I first got the Gmail account I was provided with 15 GB storage. It took me about 10 years to use 3 GB of it and I decided I would be dead before I filled up the account. There was no need to change how I had been doing things. Then the agency downgraded it to 5 GB. I'm good for a while but now I also find I can no longer see how much storage I am actually using (maybe I am supposed to be using Chrome?) I doubt I am at 5GB yet but I probably need to devise a different strategy. (I guess I could get a different Gmail account with 15GB free but then I'd have to have an email address like JohnDoe459187@… instead of my more memorable one now, plus then have to notify 100 people about the change.)

My strategy now is to move older emails I want to keep, but don't need to have access to from all devices, onto my computer. Maybe also the same for the Hotmail accounts, though those are currently under less storage pressure. I definitely want newer emails on my iPhone. (Well, readable when I am have no server contact, as they are now.)

Because I used to be able to keep everything online I haven't used computer based email software in the past 30 years or so (remember Popmail?) I am familiar with iMap and Pop but I am not clear if I can do what I want to do, namely:

1) Keep some messages available on all devices (okay, iMap…).

2) Have copies of everything on my computer, or at least archive some files onto my computer and delete them from Gmail. The archived mails can go into a folder that only exists on my computer if that is how it needs to be done.

I have used Thunderbird a bit too if it is easier to do it on that.

Sequoia on M4 mini