Externals are perfectly reliable, it’s how you manage things that matters.
I’ve had (over 30+ years of machines now) as many internal drives fail as externals - which is to say not many.
The key with externals is they lack cooling, so don’t hammer them like an internal. I have some externals that are 10+ years old. One is currently my authoritative drive as I reconfigure my setup. It sits by itself and I tacked on an old case fan to keep it cool if needed. (That said, I do prefer to not use externals, the enclosure is another point of failure, and USB connectivity can be a bit unreliable).
I’d look at your total data, consolidate it into a single drive and folder structure, and duplicate that across the drives you have, using the first one as the authoritative drive.
Then get a cloud backup like storj.io on that authoritative drive, so you get local duplication and cloud backup.