At the end of the day, it all comes down to how much money the Russians have. Ukraine is like a festering sore. It is in Russia's interest to at least minimize the impact of the festering sore before doing other things.
Considering that the west ukrainans shoots the russian speakers in the east, from areas where russian speakers living we can cosider it as the ukrainan regime was forced to stop to shoot the russian speakers .
I started to map out the possible answer of Russia .
The USA target is to bring the war to the Russian border, and destabilise the surrounding of them, bad luck for the EU is Ukraine has borders with they as well.
Now, it doesn't make sense to play with the USA game, the capital could be spent on better place.
1. Syria-iraq-iran. Maybe naval bases on the shores of the ocean ,and another inland, in Iran ? start massive scale military campaign against the ISIS , to liberate Iraq and Syria kurd populated parts, and establish the control of the Iraninan /sysrian goverment above the area, including Iraq ?
2. Establish permanent bases in Venezuela and Cuba, with military/ economical help
3. Deploy INF weapons everywhere, including kaliningrad, Cuba, Venezuela, Syria and IRan . Maybe Serbia ?
4. Give acces to China to the trans-syberian railway, to deploy troops to IRan / Iraq, and to establish permanent base there . It is the most risky. Maybe shared bases in Cuba ?
Regarding Syria/Iran, Russia got 90% of what they want out of that area now. They have a lot of areas inside Russia that needed to spend their money on building up. Why would they spend their precious money there?
Cuba and Venezuela, these places are just too far away from Russia for them to extend their power. The U.S. can always pull the hand out of our back pocket and do a regime change and Russia would lose everything.
At the end, with the economy the size of Italy, there is only so much they can do.