I actually recently just did research on this for my own purposes.

• Ardour isn’t really for MIDI. It technically supports it, and people do use it with some success, but the implementation is buggy and somewhat limited compared to other DAWs. If you’re primarily doing recording and working with audio clips, though, Ardour works great.
• LMMS only does MIDI, and basically doesn’t handle recording audio at all. The interface is known to be a bit archaic. I don’t know exactly how it compares feature-wise to other DAWs. It might be great if MIDI/synthesizers is all you want to do.
• zrhythm looks really good, but unless you wanna pay for an installer, building it from source is a wee bit of a pain due to dependency version issues. There is an AUR package, but it failed to build for me.

Those are the FLOSS options that I know about. There may be others, I didn’t really look at them. They would be even less popular than the ones that I’ve listed above.

In the end, I chose REAPER. It’s not free, but the trial period is effectively indefinite (the software does not cripple itself after the 60 day period officially passes). The personal license is quite reasonably-priced ($60 USD), and the commercial one ($225 USD) is still not too bad if you want to use it professionally. It’s a one-time purchase. You don’t need new licenses for updates. It’s Linux-native, and it does all the things.

As far as plugins go, the FLOSS software has got you covered. Zynaddsubfx is kindof famous, and Cardinal is a really good modular synth (inspired by VCV rack, which is also free and available). Vital is also quite reputable. There’s a whole pile of things available: https://wiki.archlinux.org/title/List_of_applications/Multimedia#Audio_synthesis_environments

As far as installation goes, all of this is available from either extra or the AUR.