I learned on youtube

Learn by doing, I used iFixit to replace my Pixel 3 screen/body, Pixel 6a screen, and a bunch of other mac fixes. You just get better from doing it.

There is no way of getting not worse battery than the original one, manufactures try really hard to earn the best lithium possible for their new releases. Today I have a big problem to buy a very very popular BL-5C but having at least 50% of original capacity. Niche batteries typically impossible to find at all.

Messing with iShit can be very exhausting because everything is locked, you are got to have some really expencive programmators from your local wizards to move towards repairing those.

I am amused of why are you opening the thread with expressing your love to smaller phones, not your love to soldering under microscope and messing with BGAs and repairing all neighbours' Nokias after your school classes, so I am almost sure you are a wrong person to do this kind of job no matter how hard you will try.

Don't afraid of leaving your phone into hands of professionals, they typically have too grinding schedule to read your iCloud files or harm you using the hardware vector.