Having an issue with a 2017 Mitsubishi Lancer SE (2.4L, 85k mi, Automatic Transmission) that I believe is either battery or fuel pump/injector related.

When trying to start my car, If I turn the key all the way it will drain the battery to death and I get a clicking noise. A battery jump restores it fine. I can turn and hold it 3/4s of the way and it will slowly crank up, sort of like when dudes from the WWE gain momentum and break free from a headlock.

I had my battery tested by two stores and an old school mechanic who all said my battery was fine. It’s two years old and I wash off the corrosion any time Ive seen some. Don’t remember the exact reading but in the green and hitting 14v on start up I think?

They all seemed puzzled but one of the shop techs told me it could be a fuel injector or pump causing the battery to short by not pumping properly on start up. I’ve noticed my mpg has dropped noticeably, though not sure it’s directly related.

I’m balling on a tight budget so have to fix it myself if possible. Should I do the battery first or go for the pump/injector, and are the latter something a semi-novice could handle properly?