I have a KX3 and I use a diode soldered in line between two sets of Anderson power poles - no switch, clean and simple. The unit is then covered in heat shrink. Its tiny and easy to use. I use yellow heat shrink and put an arrow on it in the direction of current flow. Don’t put it in backwards - radio won’t turn on!! I have two in case I loose one.
When my Lipo is fully charged I put it inline and get about 15.8 volts on the KX3 (on the menu for BATT voltage). There is a voltage drop across the diode and also in the KX3. Lipo = 4 x 4.2v = 16.8. Then the diode drops 0.6 volts = 16.2, and the KX3 seems to have a little drop down to 15.8volts.
I say that I can remove the diode but in actuality I rarely do - I just keep it in. I believe Elecraft recommends 13.8volts for 15W, although it works down to 12.8 - just what I’ve read not tested personally.
So if you think about it when each cell is discharged to 3.5 volts = 14 volts - 0.6 diode = 13.4 volts. The KX3 probably reads 13 volts because I have noticed a small voltage drop. This is just about right for you to operate at 15W even with the diode inline with the battery almost drained.
Simple is best - so the only issue is the wasted power across the diode. I use a 1400 mAh battery and I have never drained it even with 1 hour of operating. Within that hour I am calling CQ a lot! I always use 15W SSB for my SOTA activations.
You should use a battery monitor plugged into the cell voltage sensing socket of the battery pack. I use the kind that beeps at a set voltage and the voltage at which it beeps can be adjusted.