My recommendation for everyone is to use Fastmail and a custom domain.
Fastmail is extremely reliable, and since they charge money they also offer customer support. A few years ago I lost a lot of emails due to a client bug, and Fastmail support was very helpful recovering them from backup.
Use a custom domain so you can change providers in the future so you're not locked into your provider and can change if you aren't happy with them anymore.
I think Signal is trying too hard to be secure, and they are missing a lot of convenience features.
For example, you can't migrate from Android to iOS or vice versa. You'll lose all your messages and groups.
Or it's really hard to export all the photos someone sent you. I get that my iCloud library isn't as secure, but I really want to make sure I don't lose those photos if I lose my device. The photos aren't that sensitive.
The security notifications are also well-meaning, but hard to make sense of. You sometimes get notifications for changed security numbers, when people change their phone, sometimes folks show up twice in groups, etc. It's all a bit hard to understand and difficult to use.
And finally, it seems that messages are sometimes delivered a bit unreliably or with a long delay.