Thank you for the input!
To answer some of the questions - The Advocate is an Advocate70. I do have another (even more petite) DD to pass it to.
I didn't put it in the initial post, but I do have some reservations about the Radian. We have an XTSL (which I think is similar to the RXT) in the grandparents' car. I'm concerned that the tug-tug-tug of the harness adjuster will annoy DH. It also has nowhere near the side-impact protection of the Advocate, though from reading other threads on the board, it seems that RF may trump side-impact protection for safety. (Feel free to debate this.) I was thinking about the Nextfit, though that also seems to have harness adjuster issues. And I was also considering a Foonf, but it's very expensive for a little extra RF time, and I'm worried that the front seat will need to be pushed forward. (Kecia did such an amazing job with the chart in August comparing the space needed for RF convertibles but unfortunately didn't have a Foonf.) Anyway, thank you for the advice!