https://www.themoscowtimes.com/2022/05/18/shelling-hits-russian-border-village-sugar-refinery-a77711 [It's a real problem what to do about artillery. If they attack the artillery on the other side of the border of Russia, it's risking inflaming things. But they have to take the artillery out. Tricky one]
Isn't the moral response on this actually the game theory Tit for Tat strategy. See eg www.investopedia.com/terms/t/tit-for-tat.asp
The idea is that you take the moral stance the first round, ie co-operation, but if the other side cheats on you in the first round, you cheat on them in the second, and then you mimic their behaviour in each round until they start co-operating again (but never more than that, ie you do not take "revenge" over an above that). Experiment after experiment show it is the best way to get results over time.
It avoids the appeasement risk - ie that an immoral player on the other side will just take advantage of your morality, which seems to me the flaws in what might be considered the Quaker strategy (there was a discussion of the Quaker position on one of these threads). If you always take the high ground, and co-operate, with an immoral other side, you may feel moral but you will always lose.
In this case, if the Russian artillery is just massing on the border, but not firing, the moral situation would seem to be not to fire over the border (even though the Russians are only doing it to later bring into Ukraine), but if they are firing it from over the border, then tit for tat would mean UA needs to fire back, but only in response, and to the extent that such artillary is being used over the border (rather than trying to hit Moscow).