There is also Ardour which is FOSS and would be enough for recording as well as Traktion Waveform which has a free and a Pro version, if my memory is correct the difference is what kinds of instruments/effects plugins come bundled with it.

Ardour is the OG for sure! I'm a huge Reaper guy though been using it since 2009. One of the few pieces of software that I truly enjoy using, and the community is great

I have a RME _PCIe_ Raydat card that is supported by Linux. It has 4+4 ADAT ports, meaning 32in+32out channels. You can connect basically every interface that has ADAT to it. I've connected two Ferrofish Pulse8 AE, a Focusrite Scarlett and a Motu Traveler. :-) If your UA Apollo has ADAT, you still can use it this way! Of course it hasn't to be a RME Raydat, any other Linux supported interface with ADAT does it too, f.ex. a Scarlett. You could get a cheap Scarlett and connect your Apollo to it. Hahaha. Seriously, this would work.

That's awesome. I have ADAT converters as well, but the Apollo itself does not operate as standalone, and unfortunately has very proprietary Thunderbolt drivers. I'm pretty sure we will never see Linux support for Apollo thunderbolt interfaces. I would definitely go RME for Linux.