That's right! At the very least, maybe one could open a "repo" or a wiki that collects documents all rootable Android phones and which rooting method works and to what extent. As a long term project, of course.

That's what XDA is for. There's a lot less activity than there was 5-7 years ago but you can still find useful info there.

Do commercial apps work on such an installation? I have seen them get worse, checking for non-rooted, non-developer-mode-enabled, Google Play Store enabled, etc.

What phones are running Graphene?

Take one thing at a time and give it time. There is a lot to learn, depending on what level of understanding you want to achieve. How about starting here: https://en.wikipedia.org/wiki/Rooting_(Android)

Rooting: making yourself the true owner of your device from a software perspective. You already paid a butload of money to buy the physical device, but the manufacturer doesn't allow you to install and/or remove apps (packages), customize or otherwise tweak the phone to the extent that the underlying operating system would allow.

Unlocking the bootloader: to remove the restrictions that otherwise prevents you from altering (tweaking or replacing entirely) the operating system as implemented by the manufacturer.

Custom "ROM": I don't really have an answer for you here, since I don't understand - and haven't yet looked up - why people call it that. Anyway, people use it to mean "custom operating system", such as Graphene OS or Lineage OS.

Edit: "custom ROM", as I suspected, is a colloquial term and not technically accurate: https://en.wikipedia.org/wiki/Custom_ROM

Am I allowed to say, I'm rooting for you?

If you ask me, as long as you install GOS before sideloading is disabled, you will be fine, since your not going to need system updates from Google anyway. Your device will already have been made "unofficial" and unsupported in the eyes of Google, since you unlocked the bootloader in order to install GOS in the first place. And relocking it after GOS installation doesn't change that fact.

The bigger problem will be increased monopoly for Google. Amazingly talented developers won't be able to contribute to Android anymore because they don't want to dox themselves.