Russia has many active duty brigades available in Russia itself that are now preparing for deployment into Ukraine, presumably to reinforce northern Donbass and Kahrkov so as to first contain the Ukrainian offensive there and then later to regain substantial territory lost there. But it is obvious that the Russians did not have enough standby immediately ready units to respond quickly to such an offensive by Ukraine. Ukraine will likely regain more territory and might also be able to capture Lyman in the coming weeks. Russia will take back substantial amounts of that territory. That will come at a cost of time, personnel, and materiel that could have been been much reduced had the Russians had probably 10,000 to 15,000 additional well equipped troops in Kharkov and Northern Donbass already there when the offensive by Ukraine started.Russians has to make a priority list, considering the lack of manpower.
They don't have enought resources to protect the full frontline.
It looks like now they allocated the resources to defend kherson and the seaside areas.
These having the highest strategical importance, the logistic line is the longest, and if they loose it then it will take lot of time to recapture.
The areas defende thinly and give up are the ones closest to Russia, with the shortest supply routes, and from strategical standpoint the least important ones.
Easy to loose, easy to get.
The NATO forces tried to crack Kherson, but failed to do so, all of the noise that they make in north is just to force Russia to weaken the defense of Kherson and the coastal areas, by removing units from there to protect strategically unimportant areas. Additionaly any unit that travel west to east will strugle to go back to original position if there is a concentrated push at the coast.
In North they have light NATO units, in south they have heavy units. Russia blown up dams in Kherson, and pushing hard there .
I believe that the Russians will launch an offensive to recapture all territory east of the Oksil River that they have launched recently and parts of Kharkov immediately adjacent to the Russian border as well as Kupyansk. I do not believe that they will try to recapture Izium any time soon.