GML How to depth sort tiles that are bigger than one tile as one object?

I've come across a huge problem, basically, I can either create loads and loads of individual sprites and corresponding objects to then place in my room which is really not a good way, or I could use tiles. I have a tileset of grass and trees where the trees are 3x2 and made using the brush builder.

I know how to depth sort individual instances in the same layer, and tiles wouldn't be too hard using the same method, but how would I do the same with the trees? I've had an idea that I could go through the tile layer and have a script convert the tiles to instances, but how would I do it so that multiple tiles (such as the tree) form one instance?

The depth sorting code I'm using is a slightly modified version of FriendlyCosmonaut's from the wonderful Farming RPG tutorial. Any ideas how I could achieve this?
 
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