I plan to do the plastering myself, although I’ve never done plastering on this scale before. But how hard can it be
I said the exact same thing 40 or so years ago. I had done my fair share of drywall installation and compound spreading so I assumed I could do at least a fair job plastering. We had been looking at new houses and my wife was impressed with one that had an open floor plan. Rather than tear the whole house apart, I removed an entry hall closet adjacent to the living room. With a truss roof I knew I could remove the whole thing because it was not a load bearing part of the house. Removed the living room wall first.

Removed the hall side of the closet and the ceiling plaster and blueboard (plaster baseboard) next.

I chipped away the skim coat of plaster wherever the new blueboard met the old and applied mesh tape.

Mixed up several batches of scratch coat plaster. Very much like Goldilocks, my first batch was too thin and sloughed off the wall. My second batch was too thick and refused to spread. My third batch was perfect but I realized applying it to a wall was a completely different challenge from applying it to the ceiling. It also occurred to me my Goldilocks approach would involve a second, finish coat.
At the time, the Internet was mostly *********** so I searched the Yellow Pages portion of the phone book and found lots of professional plasterers. None of them were interested in coming to the house or even providing a guestimate -- it was too small of a job. The free newspaper classified section came to the rescue with an ad placed by a retired plasterer who was looking for small jobs. I replaced all the blueboard I had ruined, re-taped the joints and he arrived the next afternoon. In about an hour he had a perfect scratch coat on wall and ceiling. He returned the next day and applied a perfect, flawless finish coat. He had warned me at the start that he wouldn't do the sand finish so I put a bunch of sand in a can of ceiling paint and after the second coat it was an invisible repair. This master plasterer gave me a $50 quote for the job and when I tried to pay the man more when he finished, he was adamant and said he was already charging me too much.
The living room expansion saved me from buying a new home at the time and my previous project of turning an unneeded fourth bedroom into an expansion of the family room provided a door to reduce the noise from the family room in the living room when friends dropped by.

That last photo shows my flooring foolishness. Liane asked me to tile the floor of our small galley kitchen. My undeveloped frontal lobe suggested I do the living, dining, kitchen and family rooms in the same 12" x12" tile. To prove I was insane I did the kitchen and family rooms in a running bond pattern. A 2019 photo by the new owners of the house posted on the Internet shows the two patterns.

I have recently done some exterior plastering repairs on our current home and it came out just fine (watching a master helps). I'm sure you will make your barn expansion plaster job as perfect as what's on your house.