Another thing you could consider is using texture splatting for your grass (transparent layers on top of your tiles), which basically is the most simple kind of decal, you might take a look at this pen. I see what you mean with different terrain types and the overlapping grass, you basically have your ground texture and use 1-4 transparent tiles on top depending on the neighbour tile, it is a bit more complicated but possible and gives the desired result. Anyway good luck 
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