Honestly keep using Reaper at least occasionally, it is the most no-nonsense professional software I have ever used and it works great on Linux. Cockos deserves your money and it is an industry leading tool!

Otherwise I would recommend LLMS.

For a leftfield option check out Supercollider!

https://supercollider.github.io/

https://doc.sccode.org/Help.html

Supercollider is an elegant extremely mature open source audio programming language, it is a programming language true but it comes with a nice IDE and there are MANY plugins with GUIs that you can download and try, the point isn't really to code so much as put all the tools of a DAW straight into your hands.

I've heard Ardour is great but has a little bit of a learning curve. Once you get comfortable with it, it should work as well as any other DAW and its completely FOSS

Have you considered LMMS? I think it's exactly what you're looking for. Extensive manual, free, open source, made for Linux, focuses on doing the basics well

I've used Ardour. It works and Unfa posted a lot of really good tutorials, but even after about half a year of using it, the clunkyness (especially for workflows that focus on samples and programming synthesizer plugins rather than recording real instruments) never really went away for me. I'm using Bitwig now, which is not open source but works great and has a good workflow.

BespokeSynth isn't very professional but the most funny Audiosoftware I ever saw!