Building up window edging

[TheInsulator]

Prepping to skim a bedroom wall and it looks like when the windows were replaced (years back) they were slightly smaller than the previous and the installers build up the edging with bonding.

As I've pulled back some wallpaper, trims etc it has come off in lumps so I've removed anything flakey, but now left with some significant space to rebuild.

As it's a solid wall, no cavity, I'm thinking of layering up some sand and cement and then sticking some corner bead I to that.

Is there any better solution? Cement board maybe?

 

[Wayners]

Do you think elements board as it has some insulation and stick with foam would work?
Use plastic beads so no rust with the single skin wall.

You plastering with limelite?

 

[TheInsulator]

External insulation is due to be installed later in the summer so not too worried about trying to get insulated pb in there.

Im new to all this but was intending to just use gypsum multi finish, is that a terrible idea?