[bazookajoe]

Your situation may well be to do with filenames. To run tests on this for yourself to see if it is filename length, maybe try change your default filename template in CC. Settings > File Name > Use CC template everywhere > then enter this as template: {id}
That's the shortest name length I can think of (e.g. I'm looking at one of my books now in my experiments and the ID is 4096). Maybe just send a few down not your whole collection and see how that works out. Apparently it's not the best choice to use as an identifier but at least for testing purposes that's the shortest you're going to get imho.

In my instance I don't think that's going to be the fix, I've run some preliminary tests on that already to get filename smaller (apparently Onyx only supports 255 char) and it's under 255 but, the complication there is regular Android. I have the same issue with my S20+, Tab S6 and Tab A 8.4 2020 and Tab A 8.0 2019 w/S-Pen and across 3 different Android iterations (edit: with OTG storage, to be clear, regular extSD in my slots are fine. Storage scope is getting increasingly strict post 10 from what I gather).

I wish I understood all this better like, is there a way I can use symbolic links to change how the path is viewed? StickMount sort of does that but it's not a fix for me. My expertise stops at being able to TWRP, root and eventually smash through until I get Apps2SD or Link2SD to work back in the old days lol.