Ability to travel implies nanotechnology which allows metabolizing any resource which deterministically leads to expansion

Both of those aren't really supported. Maybe all matter will be useful, or maybe rare elements will still be preferred, and what exactly nanotechnology will look like is up in the air (it could be that life has explored the space of possible nanomechanisms pretty well already). Similarly, it's totally plausible an advanced species could stay in place. I actually worry we won't have the attention span to fund slow interstellar expansion.

I'd agree there isn't really a rational reason to cross interstellar space just to be a dick. Irrational reasons are possible, but seem unlikely. The only way dark forest makes sense is if competition is an issue, and the nature of space warfare leaves a strong pressure to strike early.