In recent Android devices, you can disable and even uninstall apps for users. But for some system apps, when you go back the disabled apps info page, the "Force Stop" button isn't greyed out like it should be anymore.

Like why is the force stop button active when the app is disabled. And this usually happens in OEM devices stock ROMs, not on custom ROMs afaik.

It's either a bug or intentional. Can't tell.

Graphene can also do that.

Well, there's a web installer (Chromium only because it's using very unsafe features of course) that is used by ProtonAOSP and GrapheneOS currently. LineageOS people don't want to add that because it's impossible to troubleshoot if it doesn't work, would make Firefox users feel left out, and because the normal installation procedure is a matter of some tech literacy.

And there are hundreds of native wrappers around fastboot (look up "fastboot GUI") and ones specifically for installing ROMs (look up "LineageOS installer". All of which break in all sorts of ways. The only example I can think of for a somewhat good one is the embedded for MiTool, the tool specifically for officially unlocked Xiaomi devices. Well, good luck getting a Xiaomi phone officially unlocked, however.

Almost forgot the thing I said "I have news for you" for. The closest thing to what you described (that is guaranteed to work) is opening the LineageOS website and blindly following instructions there. They are really specific and it would be really difficult to misunderstand.