I DO NOT know when Apple will drop the Series 5 from new upgrades, but the Series 4 dropped off the list with watchOS 11. My Series 4 had a 6 year run, and I was happy with it.
So keep in mind that your Series 5 will most likely drop of the list of watches that get watchOS software upgrades in a year or 2 (just a guess).
Is $99 worth it to your for that interval of time?
With respect to the Apple Watch battery.
Charge it everyday.
Try not to totally drain the battery, as that is one of the fastest ways to destroy a Li-Ion battery.
I did charge my Series 4 every day, and it was only in the 6th year that I was finding I would put it into low power model by 5pm to keep it working until bed time. I would say when I traded my Series 4 in for a Series 10, I was getting around the promised 18 hours a day, but only if I babied it. That is actually pretty good for a Li-Ion battery that gets used everyday for 6 years straight.