The spindles are the issue if you want more modern and usually not a DIY job to be done right I need more info and maybe by now this work is already done .

The work has not been done yet! Open to other ideas of suggestions. I would love iron spindles, but redoing all these stairs feels like a huge project. We wanted to see if we can live with them for a while longer, with just a smaller "update" or paint job.

Thanks!

We have now pivoted to redoing the e tire stair raling, spindles, and newel posts! I've always wanted horizontal. Any thoughts or opinions to guide installing horizontal railing instead of vertical iron spindles?

Please check the codes. Horizontal railing becomes a ladder for children.

What Joe said. And your stairs are not the place for a DIY to start to learn to do stairs. If it was a single straight run, that would be one thing. This isn't that.

Ultimately you'll make your choices for your current wants and needs and I did read your update about no kids now or ever, but do anticipate a future owner may well be put off by that style of railing due to safety reasons. If I planned to purchase a home with a horizontal railing, I'd lower the offer price to include the cost expenditures of redoing the railing.

You mention it yourself, "We live in Nashville in a pretty traditional brick 3-story home" so it's not like you're in an ultra-modern condo high rise in Miami which is more reflective of that style railing.

A few yearrs ago, Emily Henderson reno'd a gorgeous traditional home in the PNW with an updated railing that was stunning, reflected the overall style of the home/area and yet felt fresh and subtly modern. https://stylebyemilyhenderson.com/blog/portland-project-the-entry-staircase-reveal

I love the horizontal railings and would find that a huge positive if I were buying a house.....

Have you considered enclosing some of the stairs?