It's quite literally bypassing due process, unless I'm misunderstanding something here.
1. Will there be a trial? The fact that the assets were seized without trial, and that no suggestions of a trial be held suggests that OP (and most people here) are not advocating for trials. If that's the case, I have a hard time understanding how that could be considered "due process".
2. what crimes did the oligarchs actually commit? when was "being a russian oligarch" criminalized? there were russian sanctions that were passed, but those do not give the government authority to expropriate assets
3. any laws passed to expropriate assets sound suspiciously like bill of attainder, which would be banned by the constitution. At best it would allow you to expropriate assets of people who still have connections to putin/russia or whatever.
Reactions to war a blunt like this. It could have been avoided only by Russia.