Imagine if instead of doing something so noticeable, you secretly modified the drone firmware etc so that they do stuff like malfunction or blow up on launch if it's X days after the manufacture date or after a particular date, whichever is later.
The Russian army probably have to deal with a dozen faulty drones every day anyways. They'll figure out it's this manufacturer and trace it back to the software very quickly. Blow up on launch doesn't actually work because the explosives are not triggered by the drone. They either have contact fuses or timers on them. For the former, you have to drop the explosive from height, or if it's a timer, hope the operator didn't run away after hearing the click from it activating. The timer's pretty long too, somewhere between 5 and 10 seconds because they're expecting it to be dropped from very high up.
That said, I do think it does a bit more damage than simply deleting the data. The malfunctioning drones would have to be shipped back to be fixed, or simply abandoned. Meanwhile, this isn't a company that depends on data to function.
The worst case is if they lose the source code for their drone control software. This is fairly unlikely because every single developer would have at least one copy and some of them will be on vacation or sick when the attack happened and their computers would be offline. They might also have copies on USB sticks which are unhackable. The compiled software would be on the drones they already made so they can keep making copies from that. It'll just be hard to make updates without the source. It's also possible that they do not even produce the software in the first place. Maybe the technical expertise they got from China includes software.
Besides that, they might lose track of their accounts, so they'll need to figure out how much money they owe other people and vice versa by reconstructing them from emails and bank records. Everything else doesn't really matter. Technical designs? That's useful for the Ukrainians, but they have the people that came up with those designs and a production line that implements it. Personnel data? Again, possibly useful for the Ukrainians but they can look at who showed up to work. Door access codes? They can leave the door open and station a guard instead.