Crosstalk Solutions has a couple of videos on similar setups: Log Cabin Property and Greek Port. Good tips on how to design it using Ubiquiti’s free tools too.
You’ll need a point-to-multipoint setup with APs as rebroadcasters. Just having a powerful central antenna is no good because the antennas in the client devices are your limiting factor.
Return it. Buy a Cat6 cable and an unmanaged switch. Ethernet splitters are a scam, and Cat 8 is a cable for special applications in big data centres and nothing you own can take advantage of the ways it differs from regular Cat 6 unless you’re Bill Gates.