ELI5: Why haven't the Russian's opened up more fronts along the Russia / Ukraine border? Why have they kept to the East?

My question is really asking if Putin's war aims are to concentrate on the South and East to try and secure the four Oblasts there, and if he secured them, would he stop there or continue?

It just seems very strange to me (a Layman), that he hasn't tried to leverage his manpower strengths to make big gains across the whole of the border.

In Russian propaganda the Map that is often show is that Russia wants everything East of the Dnieper, plus Odessa and the South of the Country, so I don't understand why he has kept the frontline so static.

Apologies if I am being stupid, I just don't understand the military of strategic thinking.

Many thanks