I'm not saying that you need to understand every aspect of how something works to use it, but OMEMO provides forward secrecy - it is in the first paragraph of the wikipedia article. Delta Chat explicitly does not. Finding the right tool for your needs/expectations is important. We don't blame a hammer for failing to cut wood.

it's a super-shitty experience, both on XMPP and Matrix, and you touched only on one of the aspects. and that's not even the bad part. the bad part is nothing better is on the horizon.

so what you gotta do is put on your big boy pants, sit down and figure this shit out. here's where I made I misstep, let's figure out how to do redundant backups and seamless restore accross devices. etc.

because, this is it. there is nothing better coming, you gotta learn how to make do with the tech we have. offloading your shit to benevolent dictators and hoping everything will be fine is not a strategy,

OMEMO by design does not allow old messages to be decrypted by a new device. However, anything going forward should sync between all XMPP clients that implement proper XEPs. The server also has to support the XEPs. But if you're using OMEMO, then you should also already have the other XEPs required for proper messaging experience. Specifically, the XEP for syncing messages across clients is Message Carbons.

Matrix is able to decrypt past messages on new devices, but that's because it stores your keys (encrypted) on the server and does a bunch of funky key fetching and passing between sessions to allow message decryption from new verified sessions. OMEMO does not have this function.