Much much better!
I've been lengthening his A times and it has totally worked. His naps during the day are more "stable" - in that we can get straight 90min naps now (occasionally longer, especially in the mornings). He'll sometimes wake at the 30min mark but it's much much easier to get him back down. This has stabilized his bedtime considerably. We rarely get wakings between bedtime and dreamfeeds now and he's been, more often than not, sleep 7-8 hrs straight to his one night feed (from a 7-8pm bedtime to a 3-4am wakeup). The only con is that it can be difficult to get him to take his dreamfeed because of his deep sleep but we're playing around with that to see if doing it slightly earlier works better.
The next step is to try to transition to a 3.5 hr schedule throughout the day - right now he's pretty consistently doing it for his morning feed only.
Thanks so much for your help!