My carrier is definitely triggering the problem (since it doesn't happen with my other SIM card), and I also came to the conclusion that VoWIFI could likely be the source of it. But this isn't normal VoWIFI behavior, right? I don't think enabling VoWIFI shouldn't be resulting in these constant pings to my phone I don't know how VoWIFI works but this feels broken. Maybe there's a mismatch between my carrier and the Pixel 6a hardware. I am fairly certain that GOS doesn't touch the VoWIFI implementation, but I think it's premature to completely rule out the fact that GOS might be playing a role. Like I said, this issue wasn't present on the vanilla Pixel OS.
I had set this aside and hadn't done any debugging for a while, but (after writing the above) just plugged in my problematic SIM and disabled VoWIFI, and voila, the requests stopped happening! More interesting though is that when I re-enabled, it, my phone seems to be doing exponential backoff with these requests now rather than sending a request every 10 seconds. I'm still sitting here and watching it to be sure, but I got gaps between the requests of 10, 20, 40, 80, and then 160 seconds so far. This should be far less problematic for my battery life than the much weaker backoff (if any) that I was seeing before, where I had pings consistently for the whole night while my phone was totally unused.
Also, I totally agree that "tinkering with system apps" is definitely not a permanent solution. I'm doing it as a debugging step to figure out what's wrong so I can solve the problem. Without Rethink or some similar app or program I wouldn't have been able to tell that there were these requests happening in the first place. From there, I wanted to see if those network requests were really the cause of my battery drain. Before taking that step, for all I knew, the battery drain could have been due to a totally different cause.
Seems that I've got a full understanding of what's going on now, though! So I'm happy to mark this as resolved, though I would still like to confirm that this ISAKMP+UDP request pair with exponential backoff are expected behavior for VoWIFI, and I'm curious if GrapheneOS does make any changes to VoWIFI (couldn't find anything on the website pages about that -- seems to just say that it should work with the normal caveats about carriers limiting it).