I will master your song with analog gears

It does not. It is for mastering only, which assumes all mixing has been finalized yourself or with the producer/engineer you are working with.

It allows a fresh perspective and acts as a final 'polish' that can not be achieved if it were to be done by the mixer/songwriter. Another factor is taking songs either by multiple producers or those mixed vastly different tonally. My job is to make everything coherent and listenable!

First off it will make the final mixes louder! That is a key goal, to make it commercially loud. Everything will also be more lively and less dull, as multi-band compression enhances the subtle details. Mastering is the final step applied to music before digital and physical release.

It will! The process of Mastering raises the loudness/compression according to genre. Anywhere from -16 (classical/orchestral) to -5 (EDM/rap) LUFS. Streaming loudness is -14 LUFS, but commercial/radio releases sit above this due to loudness wars. I aim for -8 LUFS, competitive but not distorted.

All I need for mastering is one file per song (WAV or AIFF), no session files or individual stems. The mixes should be in final form, but I am fine to give feedback if you are unsure of some balances (e.x. maybe I think vocals could go up 0.5db, etc.). 44.1k or 48k works, 24 or 16 bit.

Ideally the mixes peak no more than -3db (-16db RMS) on the stereo out. That will give me enough headroom to apply my processing. If they are maxed out at 0db with a limiter on, I can only do EQ sculpting and not much more. If subtle mix compression/saturation was used, that is okay to leave on.