Place the two string chairs side by side with a loveseat opposite. Use a large 48" square ottoman or coffee table (not wood tone) in the middle.

You have nice accent colors in the rug, rock, and string chairs.

Paint the walls the same color as the ceiling.

The seeded glass light fixture is nice but looks a little small. I like the Edison bulbs.

In for a dime, in for a dollar I say

How about a work of string art to go over the fireplace?

https://www.chairish.com/search?q=string+art

This one is actually close to what you've already got using shapes and pastels but it's more original

https://www.chairish.com/product/7893080/70s-vintage-c-dunlap-string-thingi-wall-art-framed

Start with the floor. Remove everything from the room and paint the floor, the walls, the wood trim the same color. Keep the ceiling and beams as is.

Imagine those great chairs with a dark gray/blue wall paint.

And keep the hearth stone and Riverrock fireplace surround as is...if I may add.

A great Mission style lamp on a side table, and I like something like this for overhead: https://www.houzz.com/products/kira-home-alondra-20-drum-chandelier-black-fabric-shade-adjustable-height-prvw-vr~159444306

That fireplace is stunning! Once again, @BeverlyFLADeziner nails it.

Beverly beat several of us to it. That acidic yellow is all wrong with your incredible fireplace and all your wood trim. The red floor, while not ideal, is not as off as the yellow walls. Change the wall paint to something like what Beverly shows or a warm white--lightest shade of one of the stones.

While the off-the-shelf art work over the fireplace goes with your yellow walls, a beautiful abstract landscape echo the natural wood and stone elements in the room. See if any local artists or galleries show more landscapes and skyscapes.

"Spectrum In Gold & Black" by Rachel Springer, Canvas Print, 26x26" · More Info

Pathway Art · More Info

I second the suggestion for a rug with stronger colors and pattern.

You might want to continue with your original idea of a Mission-Stickley look, so here is a resource

https://www.missionmotif.com/rugs/

The wood over your fireplace is a feature that I think shouldn't be hidden with too large an artwork