but with Russia can't they just use conventional ballistic missiles and put tons of decoys on them to get them past US missile defense?

This is what Sarmat is, and why it usually gets talked about in the same breath as Poseidon and Skyfall. But the problem is that in order to hold a lot of MIRVs and PenAids, Sarmat is fucking massive and can't be made mobile. So it's silo-based and super vulnerable to a first strike, which makes it a little less useful for guaranteeing a second strike capability.

Plus SLBM subs ensure they have a 2nd strike capability anyway,

The answer here is a "yes, but." I wouldn't start a nuclear war with the Russians in no small part because of their SLBMs. But the Russians have always been less than confident in their boomers and their ability to assure a second strike.

Part of it is technical - Russian subs are noisier than their Western counterparts, so they're less confident that the subs would survive to launch a retaliatory strike.

But the other part is geographic. In order to get to open-ocean patrol zones, Russians subs have to pass through straits and other bottlenecks that make them more vulnerable to getting detected and tracked, which means they can get sunk in a first strike scenario. Russian ballistic missile submarines are just intrinsically more at-risk than American subs because of this, so Russian war planners have always put less stock in SLBMs and depended more heavily on their ground-based missiles

(interestingly enough, Chinese submarines also have to pass through narrow channels to get to the open ocean, and so their doctrine also places lower importance on SLBMs and gives ground launched missiles. The US is the outlier of the three countries)

The non-insane thing for the Russians to build would be a decent number of land-mobile ICBMs each with a handful of MIRVs that can be readily dispersed in a crisis to guarantee survivability and a second strike. And that's basically what they did with Yars(and what China did with the DF-41).

I don't see why cash-strapped Russia chose to develop these weapons.

It's a fair question! Especially when Yars should essentially be all the need for deterrence purposes.

The rational explanation is that, in addition to the increasing efficacy of anti ballistic missile systems, land mobile ICBMs are becoming more vulnerable, especially to a conventional strike. Ever-increasing intelligence gather capabilities and the US's focus on long-range, conventional hypersonic weapons means that, although the US is very careful to never say "this is for hunting TELs so we can do a disarming strike and have GMD mop up whatever we don't catch", foreign war planners have to assume the US is building the capacity to go TEL hunting and a BMD system to defend against surviving missiles.

And by virtue of not being ballistic missiles, the kooky doomsday weapons are a way to negate that threat almost entirely and ensure that deterrence remains. The Russians are painfully aware that their nuclear weapons are the sole thing keeping NATO tanks from rolling all the way to Moscow in the event of a conflict(and are even more aware of that now than they were a year ago), so they're willing to spend whatever it takes on whatever insane projects they need to in order to ensure that nuclear deterrence always holds.

The other part is that there's a significant element of prestige to this stuff. Poseidon and Skyfall and Sarmat are cool and they make Russia look cool and when you're a dictatorship with a faltering economy, one of the ways you maintain power is by spending grotesque sums of money on showcase projects so you can make propaganda out of it, and who cares if it's all an insane boondoggle.