Yeah, I guess I mentally slot road mobile missiles as more like "less effective SLBM on the cheap," at least for a country the size of Russia (not sure that is as true for someone like North Korea where I speculate there is a larger use-it-or-lose-it penalty). There is definitely more of a continuum here between "missile-soak" and "survivable deterrent"- e.g. at the limit you could, in theory, vaporize all of the oceans with nuclear weapons to kill all the boomers, which turns them into missile soaks, but at a truly insane level.

I've seen open-source estimates that the 33rd Guards Rocket Army can distribute their three divisions of mobile missiles across something like 5,000 square miles of Siberia, mostly steppe/taiga (which the 7917/79221 are supposed to be capable of launching from, again according to open source reporting). That's more than 10% of all of North Korea, to give an idea why it would be different for the two countries. Being open-source, I don't have a good estimate for the survivability of the TEL, but let's somewhat arbitrarily say 5PSI is the limit. A 300kt W87 can put 5PSI over 3 mi^2, so doing 5,000 mi^2 would be about 1700 of them, for a grid-square blanket search. That seems to be impracticable, just for one third of their missiles(1).

So I think it's more about guaranteed second-strike than soaking (e.g. at three warheads per silo you'd need ~600 missiles to soak up that many warheads, instead of the 70-odd from mobile). Which is why I have seen some people consider those missiles as more about assured second-strike than missile-soak, with hints that the Russians consider that their role. The Russian doctrine does not align exactly with the American one (2) for sure and there are hints that the Russians consider road-mobile to be different from silo deployments.

1: I'm not as clear on how much deployment space the 27th Guards Rocket Army, in the European parts of Russia has, and whether they will run into similar problems to the French wrt population centers. There is also a whole separate discussion about how much counter-force and counter-value are truly separate on the receiving end, given, e.g. if Barksdale gets nuked Shreveport is going to be very very sad. But the RAND people were sure they were distinct!

2: At least, as far as this monolingual American can tell. My main source for this is the Arms Control Wonk blog and podcast, which actually does read and report on what the Russians describe as their doctrine, they are my source for the "Russians seem to consider road-mobile as more survivable second-strike than silos."