I've been doing it for a long long time with the SD card slot

Wouldn't raid1 just make both cards fail roughly at same time ? After all you're writing same amount of data on both

No. SD + internal.

And "roughly at the same time" is fine. I can deal with that.

Hasn't happened like that but even if it did, even an extra day is all you need to save it

I guess just getting 2 different brands of cards would be fine for that.

I've seen devices like this for servers, basically 2 microsd cards with hardware raid1 mounted inside the server. It wasn't doing much writing on it tho, it was designed so you could run say hypervisor off it and be able to use all drive slots for storage.

I also have an rsync backup that looks to see if I'm on a certain network and then runs the backup accordingly.

It's sufficient. I'm not doing things that important