So, this probably won't be helpful but I'll add it on anyway, just in case. If this has already been said, I apologize, there is more than 200 answers here. RCS was working fine and effortlessly for me when I had my Pixel 4a. I had to upgrade it, due to it being end of life. I got a Pixel 7a and ever since I got it, it's been impossible to get RCS running on it. It seems to me that a lot of people struggling are using 6 and 7 series. I'm wondering if this somehow could be Tensor related? As before the 6 series, Pixels were using Snapdragons processors and not Google's Tensor.
This could be completely unrelated though.