Bangle.js 2 replacing vibration motor with a buzzer

I (finally) opened the watch and attached a 27mm diameter piezo disc (with cutting sides to fit in the case) directly to the pin D43 (UATX) and GND. Even without a MOSFET, it sounded loud enough before reassembling the watch. But after assembling the watch and glued the screen, the sound level decreased significantly as expected. It was kind of an experiment. The piezo disc was not glued to any hard surface (such as inside of the watch back) which is needed for the disc vibrate properly.
My eventual goal is listed as below.

1. Modify (redesign and 3D print) the case for bigger battery and serviceability (screws instead of glue) and hard (aluminum maybe) back cover for the piezo disc.
   
2. Add a MOSFET driver for the piezo disc using one of the pins, D42 or D43.
   
3. Move the buzz motor to side of the watch and glue to the case for better buzz feel.
   
4. Add a button switch to one of the extra pins (D42 or D43)
   
5. Unfortunately the HRM sensor will be removed due to the back cover with the piezo disc. I don't really need the HRM anyway.
   
   

I wish I had another extra pin for digital input, but I guess there is no more extra pin, is there? If there is one, I could add one more switch. Actually, if I could use 2 DI pins, I could add 3 switches using binary input.

The reason why I want a bigger battery is for my golfing (golf-gps). Because it needs the GPS on all the time, the battery lasts about 5 hours which is normally fine even if there is some delay on the course. But if there is a longer delay, the watch can't work until the end of the game.
Just an FYI, the battery usually lasts longer than 3 weeks with normal usage (clock, notifications from the phone, and occasional HRM).

I'm also very much interested in Gordon's GPS low level communication that he improved the fix time a lot. I'd like to find a way to reduce battery consumption by the GPS, power saving mode or data retrieving frequency adjustment, etc.

1 Attachment