1) Should i renew my FLR(M) in December while I"m still here? Or should I let that be the time I move back?
That is a personal choice for you, and will depend on your circumstances and schedule.
If you won't be moving back to the US then, you will need to renew it because you will need a valid visa to remain in the UK.
If that is the time you are thinking of moving back to the US anyway, you don't need to renew it and can let it lapse. In this case, just be sure you will be out of the UK before the visa expires.
If you're not entirely sure about the US and think you might return to the UK a few months later, I would recommend to renew before you leave so if you want to come right back to the UK, you can.
2) Will this set-up prohibit my ability to apply for a FLR(M) in the future? Pretty important for us
If you leave the UK for a few years, you will be resetting your UK immigration clock to zero. It won't mean you can't return to the UK, but you will need to start the process over from scratch again when you are ready to return, including meeting the visa requirements at the time (whatever they might be in a few years from now).
To qualify for permanent residence in the UK, you need to be living with your partner in the UK for 5 continuous years. So if you move to the US now for a few years, you will essentially be resetting whatever time you have already accumulated now towards that 5-year UK residency requirement.
3) Is this crazy? Have you all heard of others doing this?
I'm personally a big advocate of doing what you need to do. If doing this will, in the long run, be more beneficial for you and your family, go for it.
Just be aware that by doing so, you will be resetting your UK immigration clock and you will be taking a chance that you will still be able to qualify for a UK visa in several years, depending on how the immigration rules and eligibility criteria change over time. If this is all okay with you, then it's really your decision whether this will be a good move or not.
4) Should I consult a solicitor? It doesn't seem like people have very good luck with solicitors on here.
Generally speaking, people here haven't had amazing experiences with solicitors. We have 2 recommendations of people we trust who know the specifics about US/UK immigration, but they do typically come with significant price tags.
