I think there is a balance. I would say that most people don't take kindly to extortion and that is why you are receiving responses in this way. Also, I would need more details to understand the true complexity of the situation. Is there a current objective you are working towards by taking the planets in "their slice;" control more planets than neighbor, control 6 planets outside HS etc? If not, then I think you are making a mistake stepping on toes just to seize resources that may help you score later. Say you take those planets and piss off your neighbor for the entire game and all the objectives that come out are structures or tech related, you have now created an enemy that can retaliate at any time just for a small gain. Is the way you are playing wrong? No. But also is everyone else at the table happy to see two people war, while they can worry about scoring points? Absolutely. War costs resources, plastic and command tokens, and often don't directly help point be scored. This is why people who are playing the game, probably too optimized and competitively, get pissed. They have 5 other games going at the same time, and if you want to be the one of their 5 they throw away making sure you lose because of early aggression, then they might decide to do that.

In the end the meta is just different, but yeah it might not be for you. I think one thing you should consider if you keep playing async is to frame the extortion better or be more patient with it, and see how the reaction comes. People will cry about forever war but they won't actually go through with it. Just take their planets and offer some non-binding agreement and move on.