I'm very tempted to go down this route, but it seems to fail the "easy to install and use" bit.

I am a very happy LineageOS user. The installation wasn't difficult: you only need a supported phone and like 1-2 hour of spare time for backup, reading docs, flash and restore the apps. I suggest you to give LineageOS a try!

AFAIK even if you use LineageOS, gapps will still phone home and there is nothing you can do about it. MicroG/yalt is the most user unfriendly thing in this combo, far worse than just flashing custom rom, but it gets rid of it.

I like LineageOS but my phone (HTC 10) slowed down to the point of being unusable that I went with a different ROM.

Haven't gotten around to it yet, but I'll try and replicate the benefits of Privacy Guard using one of the privacy tools in the Xposed framework (via Magisk framework).

Another happy LineageOS user, on a Samsung Note 2 (old phone but fast enough to be my daily driver). Works very well and love the Privacy Guard fine-grained controls for apps.

Only if you're not bothered by the fact that you'll be giving up on the vast majority of apps that most people use.

Like what?

You can have a FLOSS Browser (Firefox) and eMail (K9), that takes care of almost all use cases for smartphone usage.

Not even by a long shot. What about 2 factor authentication (Authy or Google Authenticator), store membership apps, smart heating controls, mobile payments, chat apps, and a host of others that have no (or extremely shitty) web equivalents?

It's fine that your use cases are extremely limited, but not everyone is you.

Better than using dumbphone though. Another option is an iPhone, but understandably not everyone wants to go with it.