You could lock Penal troops's stuff behind the sidetech, but then the sidetechs would become automatic purchases.
And if the sidetechs are automatic purchases, there's not really any point in having them. Just factor them into the teching cost instead.
USF and UKF pulled off sidetech acceptably. Soviet's sidetech has been a train wreck from the start.
Imo sidetech can easily work for the soviet. Side tech allows you to choose if you want to invest into a unit or not. And since Soviets have so many good doctrinal option imo it is the way to go.
The player will have the option to either improve his core units with side tech or invest in doctrinal units.
The current changes to T2 (cheaper) do not make more attractive simply because T1 offers to much for too little. Instead they allow back tech solidifying T1 as the best option.
If one lock penal power behind side techs and upgrades T2 will become more attractive with out leading to maxim spam.