I have a feeling you are overthinking the Matrix key system.
- create account
- create password you store somewhere safe
- copy the key and store somewhere safe
- when signing on a new device, copy-paste the key
Basically it’s just another password, just one you probably can’t remember.
Most of the client apps support verifying a new session by scanning a QR code or by comparing emoji. The UX of these could be better (I can never find the emoji option on Element, but it’s there…). So if you have your phone signed in, just verify the sessions with that. And it’s not like most people sign in on new devices all the time.
I’d give Matrix a new look if I were you.
They could explain things better, you are right. I actually think I remember having almost the exact same confusion a few years back when I started. I still have two keys stored in my pw manager, no idea what the other one is for…
The decryption has gotten much more reliable in the past year or two, I also try out new clients a lot and have had no issues in a long time. Perhaps you could give it a new go, with the info that you use the same key for all sessions.