i root with magisk and use afwall and adaway to block stuff. maybe not the most secure but it's pretty private, at least i know there's no weird network connection happening without me knowing.
it all depends on your threat model, but IMO your idea is decent. personally i'd rather have another lineageos phone for calls, since the iPhone's phone app sucks (no call recording) and it being connected to the internet means it can be tracked by apple. also it's impossible to install any apps on iOS without an apple account. (unless you jailbreak, which is a pretty different story. your iphone 7 can be jailbroken with checkra1n which is good tho)
maybe an android phone with usb-c to hdmi? would be very easy to use for non techie family members too.