Interior shiplap inside and outside corner finishing
Issue: Looking for advice on how to finish interior shiplap inside and outside corners so that they look good and will be "easy" to get right for someone who is handy, but not a professional.
Background: new gas fireplace, new construction, would like to build fireplace surround. Fireplace is to code, inspected, etc. The fireplace is direct vent that is encased in a 72" wide and 24" deep false fireplace that goes from floor to ceiling. The fireplace itself is 24" off of the floor.
My intent is to install slate from the floor to about 48" and then shiplap the front and sides of the fireplace to the ceiling. I'd rather not use trim pieces on the outside as I think it has a better look when you don't.
Those outside corners need to be so perfect to look right that I would like some advice on how to improve my chances of success. I can't see any photos online well enough to see how they are joining the outside corners...I assume 45s, but not certain. I think I can bury the inside corners in a piece of quarter round (if I must) where they meet the wall...unless you have other suggestions.
I have a table saw, miter saw, basic measuring tools, air tools, etc.