(Note: I migrated from Lightroom to darktable years ago.)
The parallel to Lightroom’s spot removal in darktable is actually the retouch module, not spot removal (which is older and simpler). Sadly, the importer does a direct port to the older module, not the better one. I’d recommend dropping spot removal and switching to retouch.
It’s true that the retouch module can be complex… but in most cases, you can safely ignore the wavelet stuff. It’s super-powerful, but optional.
The basics you should know:
shift-click on the image to set a source point (optional, but useful to avoid adjusting it later)
control-click on an icon in the retouch module to stay in that mode, so you can continually retouch the image (such as dealing with multiple dust spots in the same go)
use your scroll wheel to adjust the size; hold down shift and scroll to adjust the feathering
you can turn on the highpass module to help identify dust spots in the sky more easily (and then turn off highpass when you’re done retouching) — this is useful if you’re not using wavelets (as you have other methods when using them)
right-click to remove source-destination pairs on the image (just like masks and other things on the image in darktable)
like most other modules, you can duplicate this one too (from the icon that looks like two overlapping rectangles in the header bar) — duplicating helps you use different modules for different targets on the image (such as one for sky, one for ground), so you can adjust each differently (such as with masking)
You should consider spending some time to learn about the wavelet features in the module, especially for skin retouching. Again, it’s not necessary to use the module successfully — but it’ll help you get better results, especially with trickier removals.
Here’s an amazing video on the retouching using the retouch module and wavelets. It’s around 40 minutes, but totally worth your time:
(Note: He’s using an older version of darktable, as this was filmed over a year an a half ago, but the technique is the same and the module hasn’t really changed since, so it still applies fully.)
And, of course, here are the official docs on the module: https://darktable.gitlab.io/doc/en/correction_group.html#retouch