[IdOp]

I can only add a few simple comments due to lack of familiarity with ready-made solutions and Mac's. Hopefully others can add input too.

Regarding non-ac WiFi, one possibility is to use a wireless NIC in hostap mode (where the device acts as an access point rather than client). This mode is not supported for all wireless devices under BSD, only some. Even where it is supported, the performance isn't always reliable, AFAIK. So if you wanted to go that route, investigate carefully first what is reported to work well. OTOH, you could just go with a commercial wireless router on your LAN which, again for non-ac, could be extremely cheap.

I'm not sure what protocols a Mac would use for sharing files. Do you want to mount a whole filesystem, with something like NFS? This may not be terribly reliable over wireless. Or do you just want to use sftp to transfer occasional files piece-meal?

Also, I think there's an inherent conflict between wanting something that "just works" and being new to Unix-like OS's. There is a learning curve, so it will take some time, that's unavoidable. But don't be discouraged, the fact that you've found this forum and asked suggests you're capable to do it! If you had an old spare PC available to experiment with BSD and Linux and gain some experience, maybe that could be a first step, and then making a decision about your project could come later but be made with greater confidence?