I just replaced the battery on my pixel 3. I also replaced the USB C port, because it was cheap and the phone was open. It's pretty simple to do, just tedious. I would recommend buying a replacement back as well so you don't have to worry about keeping the glass intact.
I would love a smaller phone then the pixel 3 but I'll stick with this for now, it's my absolute max size.
No not waterproof, but I didn't try very hard. I would sort of prefer the phone to be easier to open. I used t-7000 glue which seemed really good, the main spot I feel like is not waterproof is where the finger print reader attaches to the back panel, and the camera lense cover.
Looks like it's less then 5 bucks. I bought everything from injuredgadgets. I'm sure there are other good places as well. Can't really review the battery, it's only been a couple days, but so far it's good.
I would love a smaller phone then the pixel 3 but I'll stick with this for now, it's my absolute max size.