My 2025 Gen2 RIS is sitting in Chelsea for that same repair. A month or so ago I got "Coolant Low" and "Thermal System Failure" error messages. No obvious leaking fluid (it is blue, so should be obvious). Did a reset and it went away but then came back every once in a while. Scheduled...
Weston Automotive. They recently patched my Scorpion (Nail) and has held. $50 cash (extra for CC). They have their own jack pucks, so they have worked on Rivian's in the past.