Well . That’s what forks are for. The aim of GOS is not to support a lot of devices. Since it can archive its goals of a reasonable secure device that way. But as I said a fork of GOS would be good , or longer support from calyx. I think I got a year more out of a pixel 3 on calyx OS. But if your goal is maximum security there is no way around the fact that without manufacturers updates and to a certain extent hardware limitations, it won’t be possible to do.

But best would be a good Linux phone so we can move away from Android all together, hope we can see that soon