For minerals, the ONLY thing I use is mineral salts.
For potassium, I use potassium sulfate like you said. (dry form, premix in water, and dump into filter flow).
For nitrate, I use potassium nitrate.
For phosphate, I use monopotassium phosphate.
For alkalinity/carbonate, I use baking soda.
For iron, I use iron chelate powder.
The only thing I buy commercially is the substrate (Flourite and Eco Complete) which isn't much more costly than regular gravel anyway, so that's a no-brainer.
I also buy "GH Booster" from Green Leaf Aquariums (as well as all my dry powder ferts) since their GH booster is quite affordable and is a good mix.
Regarding using bat poop.
There isn't a reason I'm aware of that you shouldn't use small, controlled doses of bat poo except for the AMMONIA content. Bat turd reeks heavily of ammonia (we have bats everywhere in the buildings I work at that we are eradicating and the smell is hideous).
I'm sure you are aware of that, so in your shoes I'd only dose bat guano in extremely small doses, and check the tank for ammonia afterward.
As long as you can keep the dosing so simple that you show no dangerous levels of ammonia, then I think it's probably a good thing.
I would start this on a fish/invert free tank, and let the test kit tell you when you have the routine established.
Expect daily dosing to be necessary, otherwise you'd have to add too much guano in one big dump (ie, cause a harsh ammonia spike that only slowly is taken up by plants).
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