Look at Hubbarton Forge. Beautiful setting.

What a beautiful house and setting. I'm hoping you get lots of great suggestions and I will follow right along.

I do not like lighting that lights the ceiling and for sure not an 18' ceiling you need lighting in many places not just a fixture hung in the middle which BTW will sway in any air movement. I like cable lighting in this type of space to get the lighting down to human height . I know it has a bit of a contemporary look but IMO a well lit space trumps decor style.

What a beautiful place, indoors and out! A large chandelier will look wonderful in your room. I love mixing rustic and elegant.

Hubbarton Forge is great advice. Allison has exquisite and varied taste in lighting, listen to her. :)

I also like this very unique look:

And this beauty:

https://finearthl.com/product/liaison-860540/

I like tiered wagon wheel lights for such tall ceilings, but would look for an elegant variation of one:

I don’t know the exact size needed, so these are just style ideas.

Is it in your budget to consult a lighting designer? A chandelier alone will not light this space well. You need carefully thoughtout layers that take into consideration the height of your ceiling, the size of the space, how certain areas are used, and how the light interacts with wall surface vs windows. Notice in the picture that Jinx posted, they have two chandeliers, recessed lights in the ceiling, down lights in the beams by the walls, and multiple table lamps.

I think the first question is what do you use this room for and do you read in those chairs? I would consider a plan that puts reading lights by those chairs, sconces at strategic locations on the walls, and either recessed lights in the ceiling or if your budget doesn't allow then a chandelier. More important than the look of the chandelier is to determine how many lumens you want lighting that space. Also, will the light bounce up to and off of the ceiling from a chandelier hung that far down from the ceiling?

I love the lighting idea, but have you thought about a ceiling fan? Big Ass Fans makes some great ones and for our high ceilings it helps hugely to keep the air or heat at a consistent temp. If you go this route make sure you get a DC fan for quiet and efficiency. Don’t get one with a light.

Usually with a ceiling this high, chandeliers are added more for mood lighting/ambiance, adding another dimension, vs strength of its light. Just be sure of your expectations and also coordinate with lower lighting choices. A tiered chandelier might be a good option for your criteria.

Here are a few suggestions but maybe you need more than one, we can't see the entire room...

The double wagon wheel is the way to go

You can select something expected or something more unique.