Should I gap fill before adding DAR trim?

[Helmutson]

We've cladded the top half of the walls in our new ensuite Reno in VJ board. The transition between the tile and the VJ is not the best (I'll make sure my walls are straight next time).

We are going to add some DAR trim to cover the gaps. Wanting to replicate the look in the bottom two photos.

My question is whether I should gap fill first, before adding the trim or add trim first, gap fill, then prime and paint.

What have others done in this situation?

Cheers,

Leon