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For that size area you can also just buy a premix tub. Obvs more expensive £p/g but not expensive.
When we changed our fireplace surround I redid a similar size area. Before that I'd only filled picture hook holes.
It took me a million times longer than a pro - I ended up getting it "good enough" then block sanding it. But still only a couple of hours - basically 1hr either side of the drying time.
And tbh it'd been faster if I'd accepted my lack of skill and stopped the first time it was good enough, then sanded. I took this approach when I fixed our concrete porch and it went much faster. Obviously it would be great to get it right first time with smooth buttery strokes, but I don't think that's likely if you're not skilled.
It's sort of one of those jobs where if you're not skilled you need to filter any proper guides through that lense and form a strategy in step with your proficiency.
That's a good visual, I'm just thinking of the red area to make it neat. Usually I'd have a crack but unsure about the top arch where someone has butchered the existing plaster.
Stove being installed so keeping the inside as brick. Stove guy has a plasterer we can use if needed.