You're right, I feel like an idiot. I stayed up all night playing with Sprytile so my brain must be fried, sorry! I had it set on the Standard shader with the UV map set for albedo but the rendering mode was set to fade. I've set it to opaque and it works mostly great, with the exception that it did something weird to the transparent tiles (the image on the left is before, on the right is after). I'll keep playing around with it within Unity and figure it out, thanks so much!
Sure! OK, so I finally got a perfect result I'm happy with, but I'm going to play with it more before I post to that thread. I think it was just another case of me being dumb. I'm using the "Standard" shader with the "Cutout" Rendering Mode--the fatness/weirdness of the wooden bars were due to Alpha Cutoff being set to 0 (I progressively feel more like an idiot the longer this thread lasts, haha, the DEFAULT is .5, not sure how it got set to 0). I set the Alpha Cutoff to x >= .5 and it renders perfectly. I can't use unlit since I have a day/night cycle in the game, and the light adjusts accordingly.
I combined this with the technique posted about in the other thread describing the GIMP script that automatically spaces and margins the tileset and I get zero bleeding / perfect rendering. I'll post all the stuff I learn about the Unity workflow after I finish one full asset and I have a bit more experience in this--I don't want to post something incorrect. Thanks again for all your help and for making this and sorry for all my stupid questions!