You are also correct that those examples were not head-to-head battles. They were in fact head-to-back battles because Ukrainians only saw the backside of the Russians. Try the "withdrawal" tag but looking at the gear they left behind, "run-for-life" tag is more suitable.
HIMARS is not used for artillery combat rather used for depleting enemy firepower deep inside font lines, has been very effective and played major parts in the victories.
Russia's main advantage in this war is its massive stockpiles of artillery, Cruise & ballistic missiles, Iranian drones and fighter planes. They used them ineffectively to win at Severodenesk & Lysychansk and gain few kilometers but with massive combat losses. Now they are using them effectively against Ukrainian infrastructure mainly due to the shortcomings of Ukrainian air defense. Now that Ukrainians are beefing up air defense and Russian stockpiles deleting, how long it will last is not clear.
Ukrainians have opened up a new front as we speak. They have started attacking deep inside Russia with home made drones and with "Russian" saboteur groups. A large contingent of partisan groups are gearing up for attacks in Russian big cities and military installations. They are not just average terrorist groups but well trained guerillas with destructive power not known to Russia to date. They could be a greater threat than HIMARS to Putin.
Again, Russia will get few wins by attacking Ukrainian infrastructure until Ukraine beef up their air defense. Combat warfare is a different story. Russia has never won a modern day war since the defeat by Japan in 1905 and will not with its drunkenness, corruption and top to bottom command structure.
