Your slider/ glide are called window stops . They should Remain removable to service The window. To clean up the look you could remove the stops, clean up the corner, install a metal L bead used for drywall, make sure to tape the joint where the plaster meets the window frame before applying joint/ plaster compound. I would perhaps replace the window stops with new ones and be careful not to over nail or lightly screw them ( might want to countersink and pre drill them).

Please store your trim in your attic so the person who buys your home someday can have the original fabric to undo your mess.

@Joseph Corlett, LLC

lol, I've been undoing the messes of the previous 100 years of owners. All walls and ceilings had 4 layers of wallpaper with paint on top. Doors were painted over varnish with no primer, and the paint was bubbling. I bought this house for the location, exterior charm, and high ceilings. I don't believe in leaving old woodwork/trim unpainted if it clashes with your style, as long as you do a quality job and don't glob the paint on. I also don't believe in taking hats off indoors or any other antiquated nonsense. ;)

Style evolves, but quality workmanship is timeless, thus my question. I'm willing to spend the time to do it correctly, and I've already remodeled the bathroom, which included refinishing the old sash window for 20+ hours. I'm just looking for options here, but the standard sized trim is a no-go.

Can't you just skim coat it? I've done this many times. You really only need to smooth the area where the yellow paint meets the drywall. Where the outside part of the casing will go:

No need to bring the drywall compound all the way to the window stop, just close.

First, scrape the area where the yellow paint meets the drywall with a razor blade to remove any buildup or caulk. Then skim and feather it out.

Otherwise... a lumberyard that supplies builders might have more options for mouldings than a home center.

@Nick Miller I can certainly try that, but I don't think the edge will be clean, since it will be wood on the window side and mud on the wall side. A clean edge is the challenge, since there is no reveal on this window, basically. Thanks!

@Shawn Sorry, I think I misunderstood your question. I figured you were putting new casing on. Is the window going to remain painted? May I ask why you are removing the casing and wanting to mud it instead for no reveal? If it is going to stay painted, have you considered a simple, squared-off, non-profiled casing? That might look cleaner to you but still hide the gap.

The window still has a stool and apron, it's not really one of those clean-edged, deep set drywall return windows to start with. Just my .02

But if you want no casing, I think @toddinmn has the best suggestion.

@Joseph Corlett, LLC @Nick Miller

Now I'm wishing the contractor didn't tear out and break the piece of window casing without asking. I'll probably just replace that and shorten the apron height a bit to fit a 4" soapstone backsplash above the countertops. Thanks, all.

I wouldnt shorten the apron height, leave it, and leave off the 4 inch backsplash, and maybe someday, do a whole backsplash, with a nice tile.

The casing is pretty easily replicated. I would remove the stops and make them a little wider so they overlap the casing and get rid of that rough edge.

@cat_ky I'll leave the apron and consider tile. The continuous wall over the sink, now that I've removed the kitchen/living room wall, may look odd with tile on only the kitchen side. I'm looking into a faux beam for the ceiling , which may help divide the two rooms just enough.