Again, still kinda playing around, but is there a smarter way to do terrain transitions? Right now there's a lot of manual work after doing a 9-slice and 4 inner to get it to pick the next 9-slice and 4-inner that matches. 

Normally I'd do this with layers, but I'd like to get it programmatic.  Water -> Sand -> Grass -> Sand -> Water... If that makes any sense :)

Cases like this aren't a good fit for tilekit currently. That being said I have some thoughts on how this might be fixed and am open to suggestions on how this could be better supported. Is there any chance I could get a copy of that tileset (or a link to it if its publicly available) so I can use it for testing?

These do get complicated. Tiled has a pretty robust system for adapting to these kind of sets, but could be overkill if you just want to do 9 plus 4 style sets.

Here's the set. CC0 adapted from https://alexs-assets.itch.io/16x16-outdoors-tileset

Thanks for sharing the tileset! I'll continue to give it some thought and see if I can work out an elegant solution.