I think, perhaps, you’re being a bit optimistic regarding battery life. Since I got my M90-13T HAs back at the end of September, I’ve gone through 20 pairs of batteries over the course of 121 days. That works out to 6 days per set. I did have a couple of pairs that lasted 8 days, and several that lasted only 5 or 5½ days. I can correlate the longer-lasting sets with my doing less music and Roger streaming, and the shorter times with more music and Roger streaming.
As far as disposables go, one can pretty much always be safe if one just makes up a schedule. Change them every sixth morning, whether or not they’re giving the warning signs. The difference between 8 days and 5 days is $0.25/week. This calculation is based on the price I paid for 60 pairs of batteries from Amazon back in October.
I opted for disposable batteries because I worry about being away from a charger at some time, when I’m on travel for example. I always carry a spare set (or two) whenever I go anywhere. But, when my arthritis gets a bit more “mature”, I’m sure I’ll switch to rechargeables.