Besides affecting Poland, I'd also wonder how Soviet control over the Baltics would affect Finland.
I think a good way to start would be to sketch out how, exactly, the Soviets managed to take the Baltics ITTL, how the events in the area differed from the OTL. Like
@Augenis comments above, this would require a significant input from the Bolsheviks at a time when they were facing several other challenges as well.
IOTL, the Finnish Whites assisted the Estonians significantly by sending volunteers and weapons. ITTL, this would be the first thing that changes in Finland: either those volunteers are not sent, or then they are not victorious but many of them are lost as dead, injured or captured. This already creates butterflies for the developments in Finland c. 1920.
For another thing, Estonia especially going red (and Latvia and Lithuania as well) would mean that Finland will receive more refugees from south of the Gulf of Finland, and the Finnish government (and people) would feel themselves more threatened. This all might throw the Finnish political developments out of whack: the transition to a republican constitution might be delayed, and if Mannerheim manages to convince the parties for his intervention against Petrograd that was stillborn IOTL, things might take a turn for the worse. This intervention would require Allied support as well, but here the Bolshevik success in the Baltics might well go a long way to convince British and French politicians about the need for an intervention, too. The rehabilitation of the moderate left in Finland might well be delayed, too, if there is a stronger than OTL red scare in Finland in 1919-20. Trying to keep down the moderate left would then probably give comparative boost for the underground far left in Finland. At this moment, though, the far left was broken and inside Finland they would not be able to be anything more than a nuisance for several years. The Finnish refugee far left in Petrograd, the leadership of the civil war Finnish Reds, would be a more significant faction.
1920 was when Finland finally signed its peace treaty with the Soviet state IOTL. What happens ITTL? What does the red success in the Baltics mean for the Finno-Soviet negotiations in 1920? Even if Mannerheim can't have his intervention, creating a peace deal between Helsinki and Petrograd would be thornier than IOTL. It would most likely not be negotiated in Tartu, Estonia like in our history, due to Estonia going red. Maybe Stockholm would be the venue, Sweden acting as neutral ground? Quite likely the Soviet side would feel itself stronger and thus be ready to be more insistent with its demands. Finland is not necessarily getting even the OTL borders, which as it was were a disappointment to many nationalists. If a peace treaty is agreed upon in 1920, will Finland even be left without Petsamo ITTL? If the Finland that comes out of this is smaller than IOTL, many nationalist-minded people will be angry and this would breed revanchism and anti-Communist feelings over and above what happened IOTL.
Then of course there is the possibility that in their hubris the Bolsheviks do not agree to ending the state of war with Finland but go as far as to plan another campaign against Finland. I don't know how realistic a Bolshevik attack against Finland in 1919-1920 would be (Finland would not be a top priority, anyway) but if it does happen, it would be a big reset for Finnish domestic matters. Such an attack, even if unsuccessful for the Soviet side, might well derail Finnish internal developments entirely. It might even be that the fledgling Finnish democracy would not support such a shock: the Finland that comes out of it might be a nationalistic, right-wing authoritarian nation that only pays lip service to democratic forms of government.