Are you sure it would be like this? Great Duchy's status was unique in the Russian Empire, but would "Finnish Soviet Republic" joining USSR in 1922 be seen as "independent state voluntarily joining" by either it's leadership or big wide world? I mean, it isn't like Moscow had more influence over "Ukrainian Soviet Republic" in 1918 than it would have over "Finnish", would reds win there.
"Finnish Soviet Republic" is kind of a misnomer here. The Red Government itself called the country the Republic of Finland and I think internationally it would have been called a (Socialist) Workers' Republic.
Anyway, what the international community or even parts of the Finnish (Red) government think is not important, it matters what Lenin and buddies think in Petrograd. Lenin himself approved Finnish independence on the assumption that the country will later join the USSR. After Red victory, this would be quite likely. Thus, a successful example that by giving economic and military support to Red revolutions abroad will can make the Workers' State grow. And a possible cumulative effect in the Baltic states: IOTL, it was the Finnish (White) Army that sent volunteers to fight in Estonia, ITTL it will be the Reds. If Finland and the Baltic states go red before 1920, it will have a strenghtening effect on further Socialist revolutions/insurrections in Eastern Europe. Is it strong enough to bring more countries into the Soviet fold is anyone's guess, but the chances will not be worse than they were IOTL, at least.
It would not be such a stretch to say that seeing revolution checked just next door in Finland and the Baltics contributed a lot to the framing of SiOC: by removing that, we already add serious butterflies to the development of the early Soviet policies.
Finnish Soviet Republic (independent one) is less ASB than many other events of 1918 in former Russian Empire. Winner of Finnish Civil War was decided by a foreign invasion IOTL, Make German support for Whites a little weaker (keep Germany a bit more involved in some other corner of Mitteleuropa), Russian (or, rather, Bolshevik) support for Reds a little stronger (I dunno, opening Imperial armouries to them seems like a good start) and bingo.
The Civil War was decided at Tampere before the German landfall had any real importance. The German-trained Finnish Jäger troops were a big help to the Whites, but even without them the White side would have clearly had the upper hand. It would just have been a longer and seriously more bitter struggle, a one leading to much more bloodshed and a permanent divide in the Finnish society.
Assuming there is no German intervention but the Jägers are still allowed to join the White Army, the Bolshevik support should have been more than a
little stronger. Given their own position was not exactly strong at that time, either, it would be tough to say where they would have found the reserves for that. The Reds received the big bulk of their weapons from disintegrating Imperial units in Finland and by train from the Bolsheviks in Petrograd: I guess Lenin was already doing what he considered possible to help the Finnish Reds with weapons, seeing he was even sending airplanes to his friend Eino Rahja. It was not really the weapons that most hampered the Red strenght, it was the absolute lack of training, discipline and even passable military leadership that lost them the war.