Anything can be done later, it's a matter of cost/benefit and what you're willing to spend.

It shouldn't matter much if you install the necessary blocking for the future false beams. It would help if you posted a detail showing the size and material of it and how it will be supproted and trimmed.

Obviously it would be easier to install the beams before the cabinets and finishes.

Are those real ceiling beams or fake ceiling beams. A real beam will need some support or something to be nailed to. A fake (esp foam) will wait next to nothing can be installed at any time.

IMO firsst foret the 12' ceiling anywhere and for sure not in a kitchen and fake beams IMO always look fake so just add them later since they are not part of the actual structure.

If you plan to do decorative boxed wood beams it will be easier and cheaper to do them during construction.

Done all the time just have to take extra care to protect flooring and surrounding surfaces, and the furniture needs to be out of the room so the scaffolding can be rolled around the room.

Whether installed during construction or at a later time, the potential to install decorative beams should be considered in the design phase to ensure they work in harmony with other trim elements, cabinetry, etc.

One benefit of doing it after construction is the installers of the non-structural beams (assumption) can use the cabinets and countertops to supplement their scaffolding and ladders.

Fix the over height 12’ ceiling issue now, during construction. That’s the right choice. Don’t throw expensive halfasses at something that isn’t an actual solution to the real problem. The real problem is the *design* of a kitchen with 12’ ceilings.