When using my Surface Pro 5 (having enabled smooth scrolling flag), the scrolling with the trackpad is superb - even pressing space to scroll is smooth.
When I use Edge Dev on my desktop however, the scrolling is NOT smooth; is this something fixable?
That is an interesting issue. If you haven't already, you may want to contact the Microsoft Edge Insider Support site: Microsoft Edge Insider Support.
Unless the issue has been resolved by updates in the meantime.