So as the frontline gets pushed back to the national borders surely Ukraines ability to concentrate forces on occupied territories will suffer.

The national border is a line Ukraine cannot realistically cross so Russia doesn't need to focus defensive efforts there. Russia can redirect to the currently occupied territories and reinforce, whereas Ukraine needs to fortify the national border to prevent another russian offensive into liberated border territories and continue the offensive into the occupied territories.

My question is, how much is this having an effect on the Ukrainian manpower? Are there geographical reasons Russia can't reapproach from north of kharkiv/sumy? Leaving the border skeleton crewed to push the frontline isn't realistic, it risks having the whole Ukrainian flank exposed to encirclement but spreading forces along the increasingly growing lines seems risky too as Russia is just consolidating forces.