Awesome to see you working on a terrain mod! Certainly your attention to detail and skill bode very well for the project

I love the texture work, with that "snowy rock texture" that seems not even meant as an actual texture especially piquing my interest, could potentially offer some exciting contrast to elevations above the snow line not sloped enough to trigger the rock mod.
I've done my share of TM tinkering--ironically brought it up to a buddy just the other day before he pointed me in this direction, which was after the VIP TM thread got bumped yesterday

Anyway, I wish I could offer more help, particularly with regard to your seasonal ambitions. I suspect the loose ends Maxis left leave it as technically possible but to best of my knowledge it's not yet been accomplished with much success. There certainly seem to be a lot of possibilities and little-explored possibilities with terrain mods but doesn't seem to be any
"low hanging fruit" there so to speak.
One suggestion I would make is that multiple great "rock" textures are actually a great thing and doesn't require settling on just one. Not sure how familiar you are with the .ini texture table but the thing i've been most happy with my work is probably incorporating as many top-quality rock mod textures as i could into the high-altitude, high- or low-moisture textures in that table (obviously below the snow line

)
Again, wish i could offer more help but happy chat about workings of that table and the TM controller file more broadly, among other things

good luck!!