I realized I wrote weeks - the transmitter is good for 3 months and gets flaky in the last couple weeks.
I found some pictures of someone replacing the battery on the transmitter of a G6. This is.... not really practical. [1]
What would be interesting is if you could pull out the internals and 3D print a new enclosure that had an actual user serviceable battery.
[1] - https://seemycgm.com/2018/07/12/g6-transmitter-battery-repla...
The transmitter has a counter inside and will stop working after a certain day count. xDrip has a feature where one can reset the counter back to zero and I've been using them at least six months with no trouble.
Some sensors get noisy sooner than the others. I noticed they work longer during the winter when its cooler. Last one worked for 4 weeks quite good, but in the summer they stopped working in less than two weeks.