Russians have rushed in some reinforcements so I don't think Ukraine should try to push for more. This Lyman gain was because Russia didn't have enough troops there. With reinforcements sent in, Ukraine should consolidate, and then move its troops to other locations.
Its like playing a game of mouse and cat. With undermanned Russian defensive lines, Ukraine can look for weak spots where there is minimal number of Russian troops and then attack it.
One will see how rational the Ukrainians actually are. They will definitely have a fairly accurate estimate of the Russians that they faced in and around Lyman, and they will know the rate at which they outnumbered them. As for the Russians, the Russians do not play hold on to any territory no matter what. The Russians could have withdrawn a week or two ago, but they probably decided to stick around to slowdown the Ukrainian general offensive elsewhere, allow for reinforcements elsewhere in northern Donbass and Kharkov to arrive and consolidate themselves for a future counterattack.
Hindsight is 20/20, and the Russians definitely did have the resources to have had at least 15,000 extra troops heavily equipped and on the defensive in Kharkov and northern Donbass before the Ukrainians arrived, that would probably have greatly limited Ukrainian gains. The Ukrainians have done well to do what they need to do and exploit Russian lack of man power and other weaknesses in that location.