A. I was going to suggest gMail until you mentioned you already have it.
2. You should already have an Apple ID, which is actually your Apple email address (although, they did allow other addresses, I think, many years ago). Not sure how you access the App Store or Software Update without using your Apple ID info, it usually comes up with that part already visible when you need to identify yourself and then enter your password.
Unfortunately, you are not allowed as many "Apple" addies as in the past. OTOH, if you signed up several years ago, you should have a "dot"mac and a "dot"me addy. You should still be able to add at least one more 'alias'. I think I have at least five, currently. If you can log in to iCloud (another place where your addy should appear), you can access the preferences that can detail what's still available.
III. I'm not sure how many domains you have, but if you have another that
is providing decent service, you should be able to have another bunch of 'aliases'. Many domain registrants offer 10 or more email accounts.
e. I think the key to avoiding the accumulation of "deleted" messages is to use IMAP. You probably have a POP account that is about the oldest method of email and it usually doesn't communicate with any of your devices to "know" what you've deleted. Judy still uses Earthlink/Mindspring for email and we were rudely awakened to the non-deletion problem when people reported their messages were bouncing because of 'quota' problems. I'm amazed that Earthlink is still in business; it must be at least 15+ years old and doesn't even offer IMAP.

Judy refuses to end her use because her addy is published in many hard copy places that update only annually, at best. Oh well...
Lastly, check with that current provider to see how long they will keep your current addy in "forwarding" status. They should forward any messages to your new addy(s) for at least 3 months and maybe longer. Of course, no matter how long they will do that, there will still be people who won't update their contacts/info...

Hopefully, others will have some great tips for you!
