While I am working on a surviving Imperial Germany, and likely surviving A-H, I have some thoughts that might help. First, we know that Ukrainian nationalism was still quite fresh and that of Belarus not yet fully formed, so the big hurdle is getting them to more aggressively seek independence without outside intervention. I do not find any real traction in the victors to create them as states like Poland who had a lobby and of course an obvious purpose in pinning Germany becoming a proxy to raw German anger and fight Germany on another second front. It is my opinion that Lenin might actually respect their independence but neither Trotsky or Stalin would, so if the RCW goes less successfully each might get a window of independence. And you need to deal with aggressive Polish nationalism that would likely seek to steal more from each thus creating a very unstable situation that the USSR can exploit.
That said, without the Ukraine Russia loses its bread basket and ice free outlet to the world, it is seriously diminished in power and really just becomes Russia with the south tier minority even less a minority. This Russia is that much worse off for food supply and its industrialization should also be less noteworthy as it is forced to build further from waterways, ports and deeper into the cold interior. I think you get a rather irrelevant USSR playing at the level of Japan in the post-war era, potent but only to the extent no other power gets serious in containing them. The Ukraine potentially is an agricultural and even industrial powerhouse. With investment it should be a food exporter and can offer lower wage labor to draw in the heavy industry that founds a modern economy. I will say the same about Poland. The issue is access to commodities and capital, but both have enough agriculture to begin earning monies to invest in industry, enough population to an the factories as agriculture mechanizes, and potential access to the central European market to build the necessary trade links and trade. Give them a healthier German economy and both Poland and Ukraine can become serious contenders for growth and development, each will be able to also take part globally, a situation we might predict make them just strong enough to defend their independence.
Violating your limits, I have these all fall into the German orbit and doing better than OTL but not as well, after all why would Germany invest too heavily in their industry when they make great bread baskets/grocery stores and nice markets for industry? Albeit pre-WW1 better than half of German investments were in Europe, so at least I have a great power interested in developing them, investing and giving them markets, one large enough to defend them and support them. So for this challenge you need nother power to get involved, and post-WW1 I am hard pressed to have any victor truly give a damn.