The android open source project is not going to die - it's the reason any phone except iphone runs it. Even if it gets more restricted it'll still be run by developers from all those companies and opensource devs. Grapheneos team is not going to solely fork AOSP, though they'll probably contribute.
would you and your partner be okay with it it seems is the big question. It could be a bit of a burden -- but it doesn't have to be a bad idea.
I would try to focus on my and our needs before evaluating the others needs. You ought to feel fairly comfortable with the idea
as for the relationship it might just be a really good way for them to try out cohabitation in a safe enviroment. Especially if they would confide any challenges in your partner and you.
you could start with a test-period, to give yourself some options.
yes, that's what the article I linked points out