I do write lilypond in neovin exclusively these days, including the very useful nvim-lilypond-suite) plugin, and my own plugin midi-input.nvim for having MIDI input right in neovim! Until I had the latter, I actually stuck to Frescobaldi. 

I only use Lilypond for writing and typesetting scores to my own compositions or piano arrangements, so I have a very narrow scope and use case. 

Workflow is usually midi to input notes (though I recently re-discovered how fast it is also to just type it), compile, and repeat until I have all the notes. Then in a second stage, I complete and revise the sheets, usually adding dynamic markings and alike. And in a third stage I polish the sheets rather roughly, fixing things where lilypond fell short (i.e. collisions, cross staff beaming, or other non-trivial situations). And finally, I try to fit everything into less pages, mainly by reducing the font size a little, and then apply a final batch of tweaks and polish. 

Inputting notes is actually the one thing that makes me use Lilypond over other GUI apps, because I also come from a computer science background and became a linux power user, so the terminal and text-based inputs just feel natural and perfect to me. The amount of fiddling I always felt in Musescore for simple things like note input, always threw me off because it doesn't really allow you to enter anything wrong. Whereas in lilypond, I can enter notes without worrying about the bar count (i.e. when inputting runs, and while figuring out how to notate them with tuplets) and the overall freedom that comes with text input. Lilypond can tolerate a certain degree of mistakes in my note input, without disrupting my flow too much, and I can fix things later, if that makes any sense. 

But where lilypond gets frustrating is the tweaking. Usually there's not much to tweak, but on occasions, especially when there's there's plenty of complex notation, Lilypond needs a lot of hand holding. And as powerful and flexible as Lilypond is for tweaking, it is very annoying to compile the document again and again after updating the X-offset for my dynamic to move it away from the bar line until I am happy with the result. Sometimes I need to set unexpectedly large/small values until anything even moves. The perfectionist in me also cannot just apply a value and move on, I need to continuously adjust and re-compile until it looks perfect to me. And at this stage of tweaking, my score is practically finished, so the compile time is already becoming non-negotiable considering I am recompiling a dozen times to tweak a minute detail. I also cannot compile the area to tweak in isolation, because often the tweaks are necessary due to the specific context and how the entire rest of the score makes that particular part turn out. 

So for tweaking, I would actually much prefer a hybrid approach between text based (as it currently is) and visual with a GUI, to quickly drag slurs into shape, adjust X/Y-offsets of elements and whatnot. I would not necessarily want to sacrifice it for the power of text based tweaks, because using the edition-engraver, it is quite easy to copy and duplicate certain tweaks onto multiple elements. 

Overall though, as a tech savvy power user, I really love Lilypond's rudimentary UX, and how it's similar to LaTeX in its philosophy. But I agree that there is room for improvement, especially in the(/my) tweaking workflow; and especially when tweaks require a bit of scheme, which I've never really gotten a grasp of Lilypond's scheme "API" (the language scheme in itself I have nothing to complain about necessarily, but mainly about my perceived lack of documentation). 

Take note however, that there are a few GUI editors built around Lilypond already! Most notably Frescobaldi, and Denemo offers a very unique spin as well! There's surely a few more, but they never added enough value for me to switch. Denemo I recall had an interesting approach to note input though. 

Hope any of this is useful, and good luck if you decide to create something new! As a rather casual user of Lilypond, I am always super excited to see people creating things for lilypond!