One talk covered how you had an expected total number of cycles more than anything else. So you wanted to start between say 85% and draw down to 10% before charging.
The effect is "better than linear;" see Table 2.
https://batteryuniversity.com/article/bu-808-how-to-prolong-...
In general, li-ion has the properties you want: no "memory," deep discharge OK, partial charge OK.
The one it lacks is "shelf life:" they don't like long-term storage at full charge. So they ship at 3.7 volts instead of the "full" 4.2V state. (This is the old wisdom of not keeping your laptop on a charger.)
Lawn tools, for example, might have a smart charger that draws them down after a month without use.
I imagine it's upsetting to need your flashlight, vehicle, etc., and it's only at 50-70% capacity.