That's the intended purpose. Get a PR victory, and get the Russians worried enough to accept negotiating on less punishing terms for Ukraine

I think long-term this will have the opposite effect. While Russians are slightly panicking right now, they'll get their shit together eventually and push them back. And then they'll be looking to wash off the humiliation, so I expect an intensification of fighting to ensue and a refusal for negotiations. The Ukrainians have basically increased the likelyhood of their two worst case scenarios happening.

Scenario 1 is that the US and/or assorted vassals cut off future weapon supplies based on the agreement that their weapons would not be used to strike Russian territory beyond military targets behind the border. While I think it's an unlikely possibility, the Iran-Israel showdown will be a factor, as the Russians might use military aid to Iran, Lebanon, Palestine and Yemen as leverage for this to happen. They've allegedly already threatened to arm Yemen with hypersonic anti-ship missiles as a bargaining chip for the recent hostage exchange to happen.

Scenario 2 is that Russia will not accept anything other than complete capitulation from Ukraine. That doesn't mean Ukraine will be fully annexed (I don't think the Russians want to spend the next few decades policing Western Ukraine while also guarding a direct border with multiple NATO countries that have shown to be politically unstable and increasingly right-wing anti-Russian in recent years). But it might mean the loss of even more territory. Possibly all the way to Kiev, Cherkassy, Zhitomir and Odessa.

I think the delay is owed to 2 factors:

a) This is a distraction for a counter-attack in Kharkhov area (as there's been a build-up there for a while), or Zhaporozhia area (where recently the Ukrainians tried a more limited version of this kind of attack, probably for reconaissance). Most likely Kharkhov, as logic dictates, it being the closest place, that's where the Russians will pull reinforcements from. The Russians are probably weighing their options before committing troops from elsewhere.

b) This area was largely defended by conscripts, instead of regular army, and the Russians got relaxed due the limited amount of action that has been going on before for months. The Ukrainians basically acted with a blitzkrieg mentality and there's probably a bit of confusion for the Russians on where exactly the frontline is, or what the main objective is.

Once Ukrainians stretch themselves thin and deep, they will slow down and then pushed back. The Russians are probably prioritizing fortifying areas before major settlements where they probably plan to cut the advancing Ukrainian troops. This will take at least a few more days.