Use iOS dictation on macOS
No. (Not yet) I would make sure you optimize your microphone setup and then retest to be sure the results are reproducible.

My educated guess is the Apple Silicon chips in iOS give Siri and edge over Macs without T2 chips even on those, you may be right and see differences in the two OS.
The thing you can control for now is checking that your microphone for the Mac is as personal and close to your vocal cords as your watch / phone / pad / pod are.
Proper mic makes a massive difference in Siri no matter which engine you use in my experience. The ability to be close due to size and also different hardware makes the biggest difference for me.
I would probably not use Bluetooth microphone for your tests either. Hard wire is best for Siri if you can accommodate it.