I definitely have less experience than all of y’all but here’s a few things I can think of to try to diagnose the problem:
can you get access to a high speed camera to try to film the string vibrating and watch it in slow motion? They’re expensive but maybe you can find someone in your area who has one or a university that has one and would let you use it so you could see exactly where the string is hitting something to make it buzz. Another thing I’d do is compare the geometry of your fretboard to one that doesn’t buzz. Like make a spreadsheet and use a caliper to measure the distance from the top of each fret to the bottom of the string for every fret on both your guitar and on one that doesn’t buzz. Then if there’s a big difference at any of the frets, try to make the distance on your guitar match the reference guitar by leveling or sanding or whatever. If they match and there’s still buzzing, then the problem is probably not the geometry of the fretboard but could be happening at the fret you’re playing. I wonder if it’s a fret crowning issue, I’ve seen some bad reviews on fret crowning files, and maybe crowning frets the old school way with a triangle file would help. Best of luck.