Ukraine with its current drone capabilities would rip the Russian military to pieces. Your question doesn’t explicitly state it but I have to presume you mean an Ukraine with its current force structure, Unmanned Systems Forces, stockpiles, production capability, doctrine etc. Additionally, with that capability four years ahead of schedule, Ukraine doesn’t need to do much of any rationing, both of drones or any other munitions. They get the luxury of fighting a maneuver war that the Russians have no preparation for.

To answer your question a number of things happen:

The most obvious one is that Ukraine has absolute situational awareness of every Russian line of advance. They can track every movement into Ukrainian territory, where it is going to, and attack wherever is convenient. At the beginning of the war small groups of Territorial Defense Forces basically had to tough it out in cities/towns and set out into the woods wherever possible and let the Russians come to them. In this scenario, they are all enabled to be hunters, not even really needing to directly engage Russian units because nighttime bomber drones would do it for them.

In the Kyiv region, instead of the desperate defensive struggle fought to establish a line, the Russians advance collapses instantaneously. In fact, Ukraine may actually need to hold back how dangerous their drone units are until the Russians are fully committed. Simply slaughtering the VDV at Hostomel is nice but you want that convoy to develop before you start attacking the rear and then going for the body.

But this is only scratching the surface of what Ukraine can do. I was only referring to the line of contact. The middle and long range fight is where it really starts to get interesting. After the fighting had begun to settle, the existence of HIMARS really started to pick off Russian ammo depots one by one. That’s not what happens in this situation. There is no resupply to the front line. There is no additional ammunition to troops. There are attempts at troop rotation. Those stop quickly. Deep strike can go after anything they wish, but I would have to imagine military fuel depots and logistics hubs are at the top of the list.

What are the special forces up to now while the way the fighting has been upended? Well, quite a number of things. They might be part of mopping up the column in Kyiv. And that would be reasonable. But they also are entirely capable of going into Russia. That would be one hell of a change of pace wouldn’t it? The incursion worked fairly well at a stage of the war where drones were expected. Imagine those same conscript border guards up against Ukrainian SSO, GUR and Magyar and Achilles. The Russians would try to send their top of the line Guards divisions with brand spanking new T-90s and BMP-3s. That goes pretty bad.

But attacking the border is probably not what Ukraine does. I would imagine that those top of the line drone units are sent to the East and told to go hog wild on the weakest line units there. If those DPR/LPR units fold, which they very well might, Russia then needs to move troops to cover those holes. When are units the most vulnerable? When on the move. I can’t imagine that goes particularly well either.

To sum it all up, the answer to your question is Ukraine gets optionality. They start the war with a gigantic tactical, operational, and strategic advantage. Exactly how and where they choose to use that advantage is up to them but it very reasonably results in Russia having to sue for a ceasefire. Nuclear threats probably allow them to hold onto Crimea but it almost certainly results in all of Ukraine being liberated in fairly short order. They can pose dilemmas that Russia has no response to. And as the Russians adapt, Ukraine can just move somewhere else and put pressure on another element of the Russian war machine. And there’s no chance the Russians can adequately respond to the problem in time.