I would argue that the choice to render the embedded JPG is optimal from the UX of a Fuji user. We tend to use the JPG as the creative reference we want to start from using the film sims.
Do you mean to show the jpeg when you have a raw+jpeg pair? I'm planning to add some options here in the next version, you'll be able to chose if to see both at once or only one and toggle between them
RAF files contain (or at least can be configured to contain) a full size JPG embedded in them that will have the film sims baked in (as all JPGs do). You can use EXIFTools to extract them. Immich can be configured to make thumbnails off these embedded images instead of the demosaic'd RAF. I'm not sure if they are progressive JPGs but if they are I imagine it would be pretty efficient to directly read the images at a lower res and not even have to generate thumbnails (although that probably is less generalizable as a pipeline).
This is my current understanding and may be inaccurate.