Mixing is a never-ending struggle for improvement, imo, so, really glad to hear if you think it sounds decent!
I don’t use any non-native fx in mixdown or mastering, usually… Although they do sometimes work their way into sound design/instrument building.
The TUNING track is just to offer a reference frequency if I need it. Sometimes I will work around a specific frequency for neural oscillation (aka “brainwave”) entrainment effects, i.e. 40Hz gamma-wave, especially with healing music, and sometimes in harmonic psytrance as well. I use a lot of native synthesis, and often my starting waveforms are inharmonic/noise-based or vary in fundamental tuning, so it can be useful to have a reference against which to tune by ear, especially when the music is harmonically oriented. A lot of what I do is pretty atonal/timbre-driven, so I don’t always end up using the tuning track, but it’s definitely handy to have a dedicated track for it when I need it!