Stainless steel chimney, on the interior wall. Routing through wooden ceiling?

...That then probably makes the costs explode.

As always, it depends on the individual case. Actually, there is no more cost-effective solution than a rafter roof with studio trusses. The bottom chords form the wooden beam ceiling. Very good and inexpensive to insulate. In addition, large-diameter pipes, e.g. ventilation, can be easily accommodated.

... We never used those metal fasteners that are installed between the rafters... I don't even know why not...

Probably because of the costs.

regards