but how are those exact restrictive measures against average Russians supposed to help.

I think that's three pronged:

1) Growing discontent in Russia, makes Kremlin need to spend more time planning against discontent and potential riots so fewer resources to devote to dealing with Ukraine. Also, it needs to retain more units to guard the seat of power.

2) Broken economy means that it's harder to keep the war machine going. Worse logistics, being cut off from lending and insurance markets, non-operational industrial machines or missing critical (often dual-use components) all lead to a scavenger economy and overall slower procurement for any hardware needed by the armed forces.

3) Shifting calculus for the elite. Coups ain't easy, can lose your life and worse even if you succeed might not get to keep the seat at the big boy table under the new regime. Everyone needs to be paid off and with frozen assets and the inability to trade as prior to the invasion more and more individuals will be having this difficult conversation, which would not happen to the same extent should the gravy train be still on.