Looks fairly well-made inside there, and lots of components and chips to go wrong....
If you slide the battery on to the connector, the drill should still work when dismantled. Try pressing the trigger whilst wiggling wires and connectors, and flexing the PCB.
I've just tentatively split the switch casing just enough to get the clean/lube straw properly inside it, I didn't have the nerve to fully take it apart.
Out of 20 full quick trigger presses 19 resulted in full battery strength showing, one showed only one bar, operating it slowly always results in just one bar.
Putting my ear to the switch I can hear very slight arcing as I operate it.
Well, whaddya know?....
Reassembled it and.... it's now working perfectly! The battery indicator is still completely random but the trigger now works as it should.
Maybe the arcing is actually something to do with the battery level indicator.
At least if it happens again I know where to look first.
Thanks for the help and suggestions, as always they're greatly appreciated.
For stuff like that switch that's difficult to dismantle you can sometimes get away by making a bath of contact cleaner or wd40, submerge the switch in it & operate it.
I had a brushed 18v LXT Makita that would only run in reverse. Diagnosed a faulty trigger but a new one was £27!
Decided to carefully take apart the old one - nothing to lose! The contacts for the FWD/REV switch fell out as usual but I think they were the cause of the problem. Gave everything a good blast of contact cleaner, reassembled it & it actually worked!