Ultima V was the last one he actually really worked on. From then on he had a purely administrative/production/design role.
No, it's the last one he did programming on. He still designed and wrote a fuckton on 6 and BG. SI was Spector's with RG only as production/main design. 8 was supposedly RG's but by then it's hard to tell how much is his and how much isn't (though the move to Super Mario arcade and the focus on graphics over substance are definitely on him, at least in part).
Of course he was mostly uninvolved in the specifics of the designs of the Worlds, which is one of the reason they're not "real" Ultimas, and he had almost nothing to do with UW1 and little to do with UW2 (which weren't developed in-house obviously).