Chosen Solution
Do you still see battery stats after replacing the OE battery with the iFixIt battery?
I have been running an iFixit battery in my iPhone 8 I rebuilt last year (custom blue housing :D ). Battery percentage works, battery usage stats work, battery health seems to report proper maximum capacity, performance capability, and it still lets me use optimized battery charging. I have also replaced several batteries on 8 and 8 Plus models with other third party ones and on every one I have replaced, battery health stats and percentages still work totally fine. You should be good to go.
You should still get battery percentage and max capacity info after you change the battery with an iFixit kit. Just be very careful when disconnecting the battery. Sometimes people knock off battery gas gauge which results in constant restarting and battery percentage remaining at 1% no matter how long you charge it.
