It's sustainable-ish, imo. Depends on what you want mate.
Is it good enough to get us competing for roughly 4th-6th place? Yes, absolutely. We will win more games than we lose with this system, provided we have a decent striker and pacy center back(s).
Is it sustainable enough to have us challenging for more than that? Probably not, because ultimately, as you say, it relies on us outscoring the opposition because a few high-xg chances are always going to fall their way every game. And that has its limits, and is why defense-first approaches /possession-based approaches tend to win leagues and trophies, because minimizing risk wins titles - maximising opportunity, usually does not.
I have finally accepted that this is where we will remain as long as we are run the way we are - 4th to 6th place. So I'm pretty content with the system. But, others may disagree and they have a full right to.