What is the safest (from a privacy perspective) and easiest way to set up in-home cameras?
The cunning part is to have additional hidden cameras hard wired within the studs going to a second well hidden NAS box that takes the direct mirror feed from the first NAS box via a hidden diversion of the seemingly direct cable.
The intent is to have sacrifical cameras and sacrifical storage .. along with additional cameras and mirrored sacrifical camera + additional storage .. and then optional encrypted offsite backup.
Cables Vs WiFi is interesting - cables can be traced, but they are harder to remotely tap or jam.
"Safety" from a "privacy perspective" depends on your threat model, and how you feel about cloud storage. There are obviously options which don't use the cloud, which can range from cameras storing on local microSD cards, to cameras that feed to a local Linux box running ZoneMinder.
The "easiest" way would be the "camera with microSD storage" option, with options ranging from the likes of Dufy and Waze, to Reolink and others.
Aside from running your own "NVR" type box locally (or remotely/colo), if you're not happy with cloud providings from the likes of Eufy, Ring (owned by Amazon), or Nest (owned by Alphabet), a number of cameras support Apple's HomeKit Secure Video (HKSV) -- which just pushes video through an Apple HomeKit hub (Apple TV, HomePod, etc.) rather than a third party cloud provider.
Your privacy goals may differ from others -- whether it depends on who the cloud storage is with, or no cloud storage at all, whether you want wireless or hard-wired cameras, and whether you're concerned about any local storage potentially being removed from the property.
You'd need to give more information on what you're looking for, for folk to advise -- alternatively, there are more than enough sites and blogs with comparisons and write-ups of home surveillance offerings.