It looks like I'll go with a medium-sized Serednya Slaviya instead. I found this map:
It shows the portion of Ukraine that was given to Poland as part of the Peace of Riga in 1921, which remained part of Poland until the Soviet invasion in 1939 and its reunification with Ukraine. In my alternate history, the Peace of Riga spun it out into a neutral buffer state between Ukraine and Poland, and it spent 18 years independent until the USSR came. After 1945 it retains its independence in a notional way, being made a puppet state of the Soviet Union under the Warsaw Pact.
Since it so closely follows the borders of the oblasts, I'll slightly adjust it to fit the oblasts perfectly, and call those five regions all of Serednya Slaviya. I end up with a country slightly larger than Serbia in area and population, with a total military of 25,384 in the Land Forces and 2,978 in the Air Force. That's enough for five brigade combat teams; I'm thinking one armored BCT, one "Stryker" BCT with Patria AMVs instead, two light infantry BCTs, a Serbian-style artillery brigade, and a support brigade. I'm still heavily reliant on my light infantry, just like before, but I've got some heavier forces to support them, including six companies of PT-91 MBTs in the combined arms battalions and the armored cavalry squadron. That's 84 tanks for the whole army...plus 36 B1 Centauros that replace Stryker MGS in the mechanized infantry BCT.
Armored Brigade
Headquarters & Headquarters Company
Combined Arms Battalion (Armored)
Combined Arms Battalion (Armored)
Combined Arms Battalion (Infantry)
Armored Cavalry Squadron
Artillery Battalion
Brigade Engineer Battalion
Brigade Support Battalion
Mechanized Brigade
Headquarters & Headquarters Company
Mechanized Infantry Battalion
Mechanized Infantry Battalion
Mechanized Infantry Battalion
Heavy Cavalry Squadron
Artillery Battalion
Brigade Engineer Battalion
Brigade Support Battalion
Infantry Brigade
Headquarters & Headquarters Company
Light Infantry Battalion
Light Infantry Battalion
Light Infantry Battalion
Light Cavalry Squadron
Artillery Battalion
Brigade Engineer Battalion
Brigade Support Battalion
Infantry Brigade
Headquarters & Headquarters Company
Light Infantry Battalion
Light Infantry Battalion
Light Infantry Battalion
Light Cavalry Squadron
Artillery Battalion
Brigade Engineer Battalion
Brigade Support Battalion
Artillery Brigade
Headquarters & Headquarters Company
Howitzer Battalion
Howitzer Battalion
Howitzer Battalion
Rocket Artillery Battalion
Rocket Artillery Battalion
Brigade Engineer Battalion
Brigade Support Battalion
Support Brigade
Military Police Battalion
Military Police Battalion
Intelligence Battalion
Military Intelligence Company
Counter-Intelligence Company
Signals-Intelligence Company
PSYOPs Battalion
Civil Affairs Battalion
Mortuary Affairs Company
What else should go into the Support Brigade, as far as other services for the military?