> The certificate issues were more specifically related to Apple devices not liking that an accepted certificate had expired, and a new one had taken its place, and wouldn't even allow for the updated certificate to be "trusted" in place of the expired one. On Android I'd have to accept a new certificate to trust, and it allowed me to do so - although possibly this resulted in the requirement to re-login
Huh, that doesn't sound right. You shouldn't be having to accept anything -- a renewed certificate should just work, transparently, without any interruption. You shouldn't even notice it changed.
Given that a large part of the web now uses LetsEncrypt, if this wasn't so, you would've already had problems on other websites as well.
> (and if memory serves, old messages would be unreadable).
This may have been the case if you managed to lose your keys. This is quite unlikely now since (encrypted) server-side backup of keys is supported. Additionally, if you have more than one device, one of them will likely continue to have keys and can share them with the new device once it is verified.