Hello all. Author of AngelLoader here.
There were never any plans to support TDM in AngelLoader, as I figured the in-game system was adequate enough. Adding TDM support would not be impossible, of course, but would require quite a lot of work to shoehorn it into a loader that's mostly designed around NewDark games and the way they work and are structured. For example, TDM doesn't really have the concept of some external "FM archive folder", it just keeps the FMs in an "fms" subdir, doesn't it? And it doesn't really have the concept of a set of "local installed FMs" vs a set of "local not-installed FMs", but rather just "existent FMs and then one that's 'the current one'". Or something similar anyway. I'm sure there's lots more differences too.
Also, if you wanted to download new FMs, you'd have to go in-game to do it anyway, then go back out to your external loader, which seems kinda awkward. I mean unless the external loader also duplicated the download functionality so it could replace the in-game one completely... but honestly that's more than I'm willing to take on.
Also, regarding Linux, AngelLoader pretty much can't be made to run on it natively for reasons, unless you're one of the lucky ones who can manage to get it working on Wine (some can, some can't - I haven't been able to). It's tied to WinForms because of the RichTextBox (it needs to be able to show .rtf files) and .NET kinda has no UI framework for Linux, not even MAUI supports it... there's Mono but its WinForms isn't a perfect clone either... I can't move to modern .NET post-Framework versions either because the dark theming breaks. WPF has a RichTextBox but it's unusably slow. etc. It's a whole mess.
So, really, given AngelLoader has to support rtf and a bunch of other legacy Windows-y stuff for its Windows-y games, and it's designed with Thief-y assumptions, it would probably be better to write a new loader from scratch that only does TDM and can run cross-platform. You'd get rid of all the irrelevant complexity of a Thief series loader (including the need to display rtf!) and you could use Qt or something to have a nice uniform cross-platform look and all. But I'm not a C++ coder, I only know C# and just barely enough C++ to fumble out some simple things, so I'm not the one to do it.
Anyway, AngelLoader is open-source MIT so anyone can feel free to fork, learn from, or take whatever they find useful from it.