I have grown to like many of the features of Affinity Designer (and Photo and Publisher). And I think that the interface has improved (or I have gotten more accustomed to it).
But...
I still am not a fan of the flatness of the UI elements. I think a 3d-look is more usable. (Even Microsoft has found its way back to a more 3d-oriented UI with Office 2019, compared to Office 2016 which was very flat.)
For example I think that studio palettes should have drop shadows like for example the tools palette has, when it is not docked.
And I think that each palette should have some kind of border or some other way of showing what belongs to it and what not. I can work much better if I can quickly and easily see what belongs to a particular palette and I do not have to take a second look or look very carefully. That is particularly important when many palettes are grouped so that they appear as tabs. A 3d-UI-look could make it immediately clear what is a tab, what is the active tab (i.e. active palette), what is a subgroup inside the palette and where the next tab/palette group starts.
Also I think that the page should have a border (and/or a small dropshadow) so that one can make the artboard white as well without losing the information where the page starts and ends.
For comparison:
1) This is the current UI in Affinity Publisher:
2) This would be the UI with drop shadow (rough sketch):