Nice playing on the more recent stuff.
You have arrived at the (giant) chasm that separates one style of recording from another.
1) arranged, performed live and captured with one mic system (like the Zoom which is perfectly usable).
2) played part by part with a mixture of live and MIDI performances (whether they are actually played in and "fixed" or they are just programmed).
Most music that is used today is #2. Only because most people don't have the front end resources to pay for a studio, mixer, performers, copyist etc. to get a properly played live instrumental captured in awesome quality.
You might have the perfect combination as you are a violinist. Use a DAW like Logic or Cubase or whatever works for you, and record (MIDI) parts with high quality sound libraries (including background orchestral / chamber / string parts) and overlay your real live violin overtop.
So yes you need a decent small diaphragm condenser (usually used for solo violin), an interface with built in preamps (inexpensive ones like the Focusrite Scarlet series have excellent quality these days), your choice of DAW (any of them work, but everyone has their own favourite) and enough mixing mastering chops to finalize.
NOTE: the only reason that your recently posted live string performances aren't "good quality" is because they have been sonically maximized (limited) so that the resulting recording is loud "to the ear". That might be the built in compressor / limiter in the Zoom unless you worked on the recordings afterwards.
You can hear starting each piece that there is a loud hiss / hum which is the ambient room tone. It would be possible to use a noise reduction plugin that works by taking a noise "print" to reduce this noise not only in the beginning and the end of the piece but it would help to reduce the audible ambient noise all the way through. Get rid of that and they may pass as the playing sounds decent and the composition / arranging sounds appropriate for the genre.
Waves has a number that work this way but they are $$$. iZotope has the RX series which could work.
This one I have and its relatively cheap but works about as well as the WAVES ones I have used:
https://klevgrand.se/products/brusfri
Hope that Helps!