Russia is so weird, because they are genuinely very powerful, but I think it's in ways the AI doesn't understand. And since it's all arctic nothing up there, the AI also can't handle its manpower, so they literally lose manpower through existing once their armies get big enough. Catch 22; if Russia is buffed so the AI can handle it, they'll be Mughal-tier OP in human hands. If Russia isn't buffed so the AI can handle it, they never materialize as a proper threat outside of player control.
And yep, those truces you got nothing out of are exactly the kind of lessons that taught me to fanatically control any war my AI allies get into with my neighbours.

Seems like a nice and healthy Prussia though, all in all - though consider expanding your armies at this point, unless you're just keeping them separate for attrition mitigation at peace.
... And also, see if you can somehow get into a war VS Russia and nab them as a vassal for the mega reconquest CB on the entire East.
Let's see why the AI struggle with Russia:
-It has low development, mostly mediocre provinces in the early game; it's provinces only start to make money once you get manufactures.
-AI doesn't know how to build up a strong economy.
-Is surrounded by enemies so constantly at war.
-None of their enemies had any high value provinces nor can give Russia a nice amount of ducats at peace deals.
-Terrible natural trading node and AI never changes it.
-Doesn't profit bfrom being in a node that naturally grows richer.
-Free army button + high FL means that the Russians will always be spending a lot on the military.
-All those wrong religion provinces make a tasty target for conversion, conversion costs loads of money.
-Having a big army and being surrounded by low dev, high attrition provinces means that AI will bleed manpower thus having to pay a lot of ducats to recover troops.
-Early eco is so bad that, as a matter of fact, Russia tends to get itself into a debt spiral.
-Has strong natural rivals and no one that it can bullies on a regular basis.
-Terrible Institution spread means that it will be wasting points away and being behind in tech against Ottomans.
-Has a huge territory so it is much easier for the AI to have its troops walk around doing absolutely nothing worthwhile.
-Maintains a lot of forts for no reason and most of them are borderline useless when at war.
I think that covers it. Overall Russia has a lot of potential in the hands of a player but also has all of the elements that the AI struggles with in one whole package.