*) The dungeon design is too linear. It looks like a triangular spiral, but its a line with rooms to either side.
Topologically, there is no difference between a line(/ray/segment) and a spiral. Topology has a tendency to classify rather distinct looking things as technically identical, e.g. the rather tired joke of the mathematician who can't tell the difference between her coffee mug and her doughnut because each one is a torus, the mug just has a really big dimple on one side of its otherwise very thin torus.
This is one of the problems with simply calling dungeons "linear." It is actually quite hard to create an actually human-accessible, realistic dungeon space that is meaningfully nonlinear. The thing you speak of
is a tree structure, it's just a tree structure that does not "branch," or more technically, if a node has siblings, only one node of that depth has children.
But, for example, most dungeons will instead loop upon themselves, because actual buildings that people would live in and use rarely, if ever, have long branches that don't provide at least one alternate path back to the root (aka main entrance). Real building spaces almost always have numerous
redundant ways to return to start, often multiple long, parallel hallways with perpendicular connecting hallways at regular intervals.
Neither of these structures is particularly engaging. A straight line, or a line with dead-end spurs, isn't all that interesting purely from its structure. But neither is a taxicab grid; we don't make the grid to be
interesting, we make it to be efficient, both in terms of avoiding wasted space and in terms of making it easier to navigate.
It's necessarily one of those areas where realism, gameplay, and design-effort collide. At best, you can pick two: realistic layout, engaging structure, easy to design. Easy and realistic? Not very engaging. Easy and engaging? Risks realism. Realistic and engaging? Better be ready to put in quite a bit more effort.