The Soviet leadership at the time you go back to will absolutely follow through on your advice.
For me, I'd go back to say the early 60s and tell them to invest absolutely everything they can into computers, cybernetics, linear programming, etc; and using those tools for optimal central planning of the economy. Optimal both in the sense of economic efficiency and for greater democracy in the planning decisions. Get people to feel like they have a say in the economic plans and I think you short-circuit a lot of the consumerist drives. But ultimately, you just need that computing power to run a modern centrally planned economy.
I thought about telling them to make a Xi-like anti-corruption push in the 70s or so (and Andropov was working on that before he died). But I think if you get them to focus on the stuff I mentioned above a lot of the corruption might ultimately sort itself out.
I also thought about telling Stalin that he really needs to think about maybe approaching collectivization of the farms a bit differently. But I don't know what I'd tell him to do differently though. It was a mess at first but eventually the collective farms helped get Soviet agriculture where it needed to be, I think.
I wonder if there is anything one could do that would actually change the outcome in germany. It'd be nice if Rosa Luxemburg was warned and had time to gtfo or whatever but would that have changed the overall outcome in germany? I guess you could draw a parallel and say that the spartacist uprising was sort of like the July days, but a lot of the bolshevik leadership like Lenin escaped the July days unlike spartacus. And then you could say october 1917 was similar to the subsequent revolts that happened throughout germany in various cities. Reality is more complicated than a comparison though, no idea if it was really within grasp at all.
Yeah, this is the big critique of my suggestion. I don't know enough to actually state what I would tell the Spartacist to try to change history (or if it even is plausible that it could have been changed).