Install HwMonitor from CPUID and look at what the battery states are. You might need to do an RMA if it’s legit just not charging to capacity.
There IS an M.2 slot free, but it’s only large enough to fig 2242 and 2260 sized M.2 cards. I’m also unsure if it will take NVMe SSDs, so look into a SATA card. MyDigitalSSD makes a 256gb one that’ll fit for $99 US.
As far as KMS goes, I can’t help you there.