I really wish I knew what the hell is going on. Is the plan here to allow the city to be operationally encircled before launching a relief counteroffensive to roll up the flanks of the besieger. (Maps from DefMon and David Lisovtsev respectively)
I'm curious too, I'm thinking they're just trying to exhaust Russia's short-term offensive power before reverting to the offensive themselves to reduce the danger of Russian counter-attacks etc... Though I don't know if the material/manpower ratios will allow for such a strategy.

The ratio in losses of artillery platforms is still mediocre for Ukraine, even if the medium-term prospects on munitions is favorable. At some point we have to hand over some of our hundreds of collective stored M109s.
Given that the West is still needing to ramp up 155mm ammunition production perhaps the mass of M109s will be shipped once we have the ammo to give that can match the barrels shooting them. The US should also give away its older M198 155mm towed howitzers as there's no point in keep those in inventory anymore with the M777 in service for years now.

Anyway, a clear lesson of battle in Ukraine is that forces driven to rely on small-unit action and organization on the attack are also constrained to defending in the same manner. In other words, mobile defense and defense in depth suffer when detached platoons, companies, or battalions have to carry their own weight.
That's why I'm wondering about the ability to mass power in the rear areas at all. The prevalence of drones and long range artillery makes it so difficult to keep reserve or attack formations near the line of contact that battles seem to devolve into these positional attrition fights.

A Cold-War-inspired heavy division built around Abrams or Leo2 could simply annihilate a series of such groupings in detail, granting heavy initial losses to the effect of enemy artillery and CAS. But once the first and second lines were broken, such a division could comfortably roll ahead at 5-10 km/day, causing cascading failures in enemy organization. Without excellent preparation, the defender would not be able to muster and concentrate sufficient reserves to deliver a decisive meeting engagement.
This is why the US is trying to devise what would form a modern 'penetration division' to do what you suggest. I imagine though that that even were such a force available to Ukraine right now the ability to attrite massing forces near the front line will probably require a penetration division to essentially stage tens of miles from the front-line conduct it's roadmarch, passage of lines straight into the attack in order to negate the ability of artillery, drones, and CAS to attrite them as they posture in assault positions.
Something like this would take the most precise planning and coordination on the front end and the most flexible command structure to exploit any effected penetration for a break through.

The most difficult element of constructing such a heavy division, one I lack knowledge to work through, is what NATO short-range SAM systems would be available for moving up just behind the maneuver units. The second most difficult, though more a matter of training, is how to optimize between safety and availability in the divisional artillery (SPG) assets as the divisional mass advances and diffuses out across territory.
The lack of mobile SAM systems is certainly a NATO weakness though I see that being rapidly corrected with the donation of new systems by Ukraine (Skyranger for example).
As for the artillery, the current construct would allow the brigades to use their own artillery for local support while DIVARTY supports the main effort, targets enemy in the deep fight, and conducts long range counter battery fires. The future divisional model takes artillery away from the brigades and puts it all with DIVARTY which will remove some the of the flexibility current brigades have making them more dependent on DIV support, something that may be unwise in EW contested battles.