GrapheneOS still intends to support all the supported devices until EOL. The sideloading change doesn’t affect them. It won’t apply to GrapheneOS. It only applies to certified OSes and GrapheneOS is not certified because it doesn’t license Google Mobile Services. As per the rip out of the device trees for Pixels, that just makes Pixels like other phones. GrapheneOS has been able to expand it’s automation to build that device support themselves. For new devices, making the support will take longer than it did in the past though, but they will still support those Pixels, as long as they meet the hardware requirements and still allow third-party OS support with all security features intact. Besides that GrapheneOS is actively talking with a major Android OEM right now in order to help them reach the security requirements for a subset of their future devices. They are very optimistic about tha

I would imagine the first thing any custom ROM would do is bypass Google’s app restrictions.

Those restrictions don't apply to custom ROMs. Yes, it's clear Google is trying to kill custom ROMs but I think we still have couple of years. Linux phones are improving fast and I think in 5 years we will end up in the same spot we were with PCs 20 years ago: you will be able do most of daily driving on a Linux phones but some apps just won't be possible to run (Authenticator apps, banking apps, Whats App, Android Auto...). Dual booting will not be possible so most probably I will end up with two phones: daily driver and work/car phone.