For backup check out SMS Backup/Restore. I have 10+ years of sms backup with it, all readable as text or using Excel.

I’ve never found a good solution to this SMS problem - there’s seems to be nothing out there (probably because FOSS devs think SMS needs to die, and I agree).

I did find my solution last year: JMP.chat - I think they’re considered a virtual cell provider. Port your number to them, then all your SMS/MMS gets piped into XMPP, which you can access with an XMPP client on any device - Gajim on Windows/Linux, Cheogram and Monocles (plus others) on Android, Snikket and others on iOS. My SMS works even if my phone is off. Prices are really good, so good that I use a different SIM in my phone for a data connection, as I no longer need an SMS/voice connection (calls are routed via VOIP in Cheogram/XMPP).