Once you have worked out how long you can fly until your battery hits 3.5-3.6v PER CELL, set a timer on the TX with 1 min less on it than the time to flat of the battery, that way you have a reserve.
Most decent fixed wing ESC’s should come with over discharge protection or can be programmed for it, they pulse or cut the throttle to a set % when the voltage hits 3.5per cell. That will protect your battery.
A better charger is always an investment, it allows you to use storage charge functions for days you can’t fly and will prolong the life of your battery. Personally I would rather spend $30-50 for a better quality, higher output charger. The B6 is just about OK but it is well known for QC issues with fakes and has a relatively low power output so suffers a bit with larger size or cell count batteries.
The charge rate of the battery is determined by its capacity, you should only charge at 1C. For example 2200mah battery would be charged at 2200 divided by 1000= 2.2A.