by the way it's on libgen if anyone wants a pdf.
EDIT: here it is https://libgen.pw/view.php?id=1173564
I'll try to make a drive and upload to there so it downloads faster (I think):
https://drive.google.com/file/d/0BwdaNTVtXguAbG80d1FsbEp6ZTg/view?usp=sharing
You don't need 100A.
https://www.amazon.com/Adjustable-Switching-Regulated-Interface-Alligator/dp/B08MQ8K7L5/
You can adjust the value. You should always have a way to unbrick an ECU or a spare ECU that works in the car you're tuning before you try to tune.
"Stage" doesn't matter, you can do custom recoding of the system and use the same power supply.
Not Subaru specific, but Greg Banish's Engine Management Advanced Tuning book is a great starting point. I would also recommend learning about engines (in general), Internal Combustion Engine Fundamentals is another staple to get started. You can find many online resources by searching for FSAE <topic>, like FSAE Intake Manifold, or FSAE Exhaust.
i dont know if you intend on using a phone app to do this but... have you tried some (android) automation app like automate? personally I never used it. but i know that it is possible to automatically open an app (like the one you are using) (and probably unlock the device screen) once your phone connects to your obd2 adapter.
It's an older book now but it still has a lot of good theory with practical applications. It helped a lot with understanding my K20 with Hondata.
https://www.amazon.com/gp/product/0760315825/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Better off buying this one:
https://www.amazon.com/Modify-Automotive-Engine-Management-Systems/dp/0760343454/
Same exact book, just 9 years newer.