Possible Meta offer but starting at Linkedin soon

I’m starting a job at Linkedin soon as Sr SWE AI, but got contacted by a team in Meta for Research Scientist position. I already cleared the Meta RS interview recently - but things didn’t get to the offer stage since the initial hiring team wasn’t best fit in terms of projects. So, I don’t need to go through the interview again now, but just evaluate team fit

At this point, given I’m starting soon at Linkedin, I’m not sure I’d take the Meta offer. I’d rather like to see how things are at Linkedin. However not sure what’s the best course of action here. So far I haven’t told the Meta recruiter about my new role at Linkedin - for them I’m still an Amazon employee

Best scenario for me would be to get a Meta offer which is valid for e.g. 1 year, and decide in few months whether taking it, after seeing how things are at Linkedin

My question is: can I be open with the recruiter and HM about that? Or is it better strategy to just try to get an offer, and negotiate a long validity duration (ideally 1 year) only once the offer is on the table?

The thing is, the HM will likely ask when I would be available to start during our introductory call. If I say I want to wait ~6 months before accepting, I might not get an offer at all. If I say I’m available to start soon, how can I negotiate long offer validity later?

TC: 380k
Yoe: 8