I did the same - upgraded from Mavericks to High Sierra - and was also wondering about the low free memory (also about 300 MB). I think it’s because of “inactive” memory. When you close an app, OS doesn’t free up the memory that it used, but rather marks it as inactive, so that if you relaunch the app it will load faster. If another app needs memory, but there is not enough free memory, it will free up some inactive memory to make it availble for the newly launched app.
Found a question about it on stackexchange: https://apple.stackexchange.com/a/4323
Btw, if you use iStat Menus, you can check a box (in iStat Menus 6.0 at least) under the memory section “Hide inactive memory” and it will show only “wired” and “active” memory used, so you get a better feeling of how much memory is available.
I’m at 1,003 cycles and my battery health is at 61%. I’m gonna change it asap.
In Denmark, the cheapest Apple authorized repair shop charge you almost $600 to change the battery - definitely NOT paying them $500 for an hours’ work :-P