What degree of aggression against the EU would spur the EU into action?
Russia already can't take over Ukraine. It would not be able to fight on more fronts, and certainly not against a strong EU with many more resources to spare.
Cyberwarfare and individual assassinations, sabotage, and disinformation campaigns were not enough yet. The EU is patient, because there's nothing to win. But the invasion of Ukraine showed that the EU can suddenly and significantly oppose Russia when certain lines are crossed.
Surely, it's in the interest of Russia both as preparation, but in large part to try to unsettle the EU, mainly its citizens. Much like it tried with Nuke warnings. GPS disruptions across borders. Gas and oil cutoffs. Blocking Ukraine food exports. Etc etc.
I really wonder about this too. If Russia destroys those undersea cables, would that get a direct response? I would like to think so, because that's a planet-scale disruption (I think?), but it really depends on the people in charge of the countries involved and their stomach for violence and escalation.
I'm passionate about minimizing war and I seriously hope we never fire nukes at each other again. But a country willing to inflict global damage as a kind of tantrum over their failures in a conflict they single-handedly started...I mean, we can't tolerate that as a species. There's gotta be a line somewhere.
I don't think an attack on undersea cables would spur aggressive/invasive action. I would expect more direct, obvious, and public military presence, observation, and possibly isolation of Russia on it's borders though.
Human, especially civilian lives are probably a line. Sabotage that would be inconvenience rather than disabling and cause direct suffering - I don't think it'd spur direct military action. Political - sure. The EU is already driving a hard sanctioning course, and would likely increase such political responses.
There are alternative internet routes, and we're not that dependent on them - in a fundamental, life or death or direct human suffering kind of way.
Sandboxing Russia likely won't happen without some form of reaction. Look at the conflict in the "Spratly Sea"; where Philippine ships literally get pushed from their territory, people died already; now imagine the scale of a whole country and not some WW2 ship turned to island fortress. The civilian reaction alone (aka smuggling) would put several continents into instability.
Realistically the writing is on the wall here. Either Russia comes to its senses, or it will be dealt with eventually. Yes that will be painful, but it will be less painful than letting a nuclear terrorist dictate global norms.
The reality is that if Russia was willing to walk down the latter path, they would have probably done it already.