Does an SM's RCP date supersede their ETS/EAS?

I see. Now how exactly would that play out if a SM hits their RCP? I had a SPC/P in my shop reup till 2021 but he is at about 7y7m TIS and I don't think he is gonna pick up E-5 anytime soon.

PFC (Join to see) if you look at the In/Out calls message, you'll see a note that says that a SPC (P) in a balanced or under strength MOS has an RCP of 10 years.
The RCP date is the date the Career Counselor is not allowed to extend the Soldier past. If a Soldier hits their RCP they have to ETS but are authorized separation pay. If a Soldier's RCP changes because they are reduced in rank, then there are different rules that apply.

Understood, I read the most current in/out call, and for 25u at the moment, it is overstrength if I'm not misreading this. And it is also my understanding that for SPC in an overstrength MOS, the RCP is 8 years. How often do these update?

PFC (Join to see) you're reading it correctly. On the day the SM was extended/reenlisted the MOS was balanced. How often it changes is whenever it changes. Sometimes twice a year, sometimes five times a year.