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.
Selective Sync is the one feature that Resilio provides that I use.
It enables me to grab any file, using any device, at any time, from anywhere, over any network, simply and quickly. I really wish Syncthing had this capability. Oh well.
So if I'm traveling, I can download a movie from my library with my phone or iPad while connected to hotel wifi. The Resilio UI is simpler than turning on Tailscale, launching a file explorer connecting to my server, then copying. Plus it's a robust sync job - I don't have to think about it, if the network goes away, Resilio will pick up the sync again when it can. On my mobile devices, Resilio is only run if started by the user, but Syncthing runs all the time to ensure stuff like photos, downloaded files, Backups, etc, are sync'd to my server.
I switched from Resilio to Syncthing for everything else (mobile devices mostly, since I can use other tools on laptops), because it's much lighter to run. Resilio is hell on mobile devices if you have a large library, as it keeps the index in memory, while Syncthing uses a file-based approach for indexes. Resilio is also resource intensive on my server - again because of the large media library.