I'd like to go this route as well. I've got a domain and Fastmail checks all the boxes for me, but inertia is a powerful thing.
What I ended up doing was registering for my new email and only using that for new accounts. Then I switched over my key accounts (GitHub, domain registrar, hosting, and the like). From there it's been more than a year of just switching accounts over as I notice that they're still on the wrong address.
It's not urgent yet, so I can take my time and do it gradually. By now most of the important ones have been migrated, what's left is mostly a few forums that I've not used in years.
Don't forget you never own a domain name, be careful which registry is behind the extension you use. My recommendations would be:
- don't use a ccTLD (2 characters TLDs) unless it's your country's TLD, or a country you can trust. ccTLDs don't follow the ICANN rules but the rules of the country. As a rule of thumb, keep in mind that the poorest the country is, the less their infrastructure is reliable. If you rent an exotic .so, don't be surprised if it doesn't resolve one day. There's also a risk that you lose eligibility to your domain overnight. Usually ccTLD registries tend to loosen the requirements (.fr used to be only available to French people for example, not the case anymore), but the opposite is possible. UK residents also lost eligibility to .eu after the Brexit. I'd also avoid geoTLDs (.asia, .berlin, .paris, .cat, ...) because of the eligibility rules.
- (new)gTLDs are not all reliable either. Stick to the core gTLDs if possible: .com, .net (Verisign, even with their price increase they're probably the most reliable entity), .info (Afilias), .org (PIR, the shady selling deal they had with Ethos Capital has been rejected). There are reliable newgTLDs if you trust the entity behind it: .app/.dev (Google...) for example. I also like [Radix] and [Donuts] as entities.
- don't forget to enable autorenew and to keep an up to date payment mean on your registrar. If you want to retrieve a domain name you lost it will either be costly if you're in the redemption period, or extremely costly if its "drop caught" (from "drop catching", not sure if "drop caught" is a thing) and resold on an aftermarket platform.
[Radix]: https://ntldstats.com/registry/group/Radix-FZC [Donuts]: https://ntldstats.com/registry/group/Donuts-Inc
However, if you have a large IMAP mailbox and you are locked out out if, you will not get that historical data if you are suddenly locked out. So the periodic archive download might be a reasonable suggestion.
This is exactly what I do for the handful of domains I pointed to fastmail. I use Thunderbird to keep everything local and use 7zip to make snapshots of the local files and copy them to other local servers and USB drives. If AU started imposing some weird laws on Fastmail I could quickly migrate elsewhere.