I've got a cheap and cheerful LG K8, 95 quid.
Connectivity is pretty good, regularly get the full 288 readings per day.
Looking back over the last week in history, it's been 284, 281, 288, 288, 279, 287, 288.
In "Settings/Less common settings", I've ticked "Aggresive service restarts". Looking at the events logs, that kicks in periodically.
In Bluetooth settings, I've ticked the boxes in the pic below.
I don't know anything about the ins and outs of bluetooth, so I basically just ticked anything which looked like it was aimed at maintaining connection!
A lot of people seem to have had issues with Samsung phones - xdrip sometimes pops up an alert saying "buggy samsung - using workaround" but that seems to be more an issue with weird samsung bluetooth implementation than the transmitter or app.
Another biggy is if people are running another bluetooth gadget in the vicinity.