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Spend a bit of time reading through and searching old posts. Your answer may come a bit clearer.
I find it funny how you need disk brake rear to be engineered. Considering the AE86 was available new with a 4A-GE and drum brake rear. Also consider a lot of bigger cars have drum brakes. Huge trucks have drum brakes on the front!
The cheapest way to get disk brakes on the back, if thats what you really need... is to get disks and calipers off a R31 skyline or Corona. And make a custom bracket. You then have a handbrake cable issue.
You can get the AE86 rear disks new (RDA) and second hand calibers, and make a custom bracket.
Lots of options... I prefer the full AE86 disk brake setup, but it means getting different diff housing and axles and everything.