Agreed that the invasion of Kursk Oblast is a bold decision by Ukraine. Rumor has it that the UA has pushed numerous brigades into Russia proper, with numbers hovering somewhere around 2 divisions' worth. As for the goals here? There's a lot of possible benefits, with the obvious drawbacks being as you state - the eventual cutting off and piecemeal destruction of the UA units now in Russia.
Some of the benefits, IMO, include the following:
- Territory to be used in future negotations.
- Morale boost for the Ukrainian people.
- Proof to Ukraine's allies that she's still in this fight and able to maneuver. And evidence that their support is being put to effective use.
- Forcing everyday Russians to deal with the fact that Russia isn't safe in this war, either. Not only, but any RU soldiers killed in Kursk Oblast are likely conscripts, not volunteers. These are friends and family members of people living in St. Petersburg and Moscow, not a bunch of poor, asian-looking volunteer soldiers from the far east of the country.
- Reductions of pressure from Kharkiv and Donetsk. Russia will be forced to pull troops away from other areas of the front to deal with this incursion.
- And finally, the timing of the counter-invasion (is that a thing?) is of particular note to me. The Russian's offensive near Kharkiv has stalled, and as of a few days ago there was some leaked intel that the forces in Vovchansk may end up having to pull back to stronger positions. In Donetsk, the Russian summer offensive has been going strong for months now, and is probably somewhere between 4-8 weeks before it culminates. Ukraine's strategy for the last year has been entirely focused on defense, and punishing Russia with high attrition. I suspect someone in Ukrainian command asked the question - how can we keep Russia attacking in earnest after the culmination of Russia's summer offensive, so that they don't enjoy the fruits of an operational pause and possible negotiations for the loss of Ukrainian land? And the answer was clear - put Russia into a position where it HAS to keep attacking Ukraine due to internal political pressure.
That last one IMO is the key for the decision to invade Russia. All in all, you have a Russian army whose offensive is approaching culmination - they're surely reaching a point where units are exhausted, needing to pause to rearm and regroup, and suddenly Ukraine launches a major assault into Russia itself.
I don't think the attack will reach Kursk (unless Russia entirely fails to respond) or anything. I believe this is more about continuing the existing strategy of attriting Russian forces down.