building our 18 day itinerary

That's not a whole lot of time for covering quite a bit of ground. I'd probably cut Rhonda, give that time to Madrid, and take a day trip to Toledo from there.

I think you're a bit light on time in Seville and Madrid (assuming an interest in the art museums and a day trip to Toledo). Maybe you'd rather make the shorter visits in order to have time to include places like Valencia and Ronda. But Valencia is costing you a flight you wouldn't otherwise need.

I'd plan to drop the car in Granada and just take the train to Madrid (in less than 4 hours) rather than driving to Seville and switch to a train there.

Based on our self-drive travels to Spain in September of 2019 and this year, planning 4 nights in Barcelona and 3 in Seville seems good. We have not been to Valencia or Madrid. We found Ronda very touristy and crowded to the point of unpleasantness so a half day there was enough. We did, though, enjoy the small nearby town of Arriate and walking the El Caminito del Rey was amazing. Granada was also very crowded and, for us, two nights were plenty to visit the Alhambra and enjoy good food. Zaragoza is interesting, especially the fabulous Aljaferia, and nearby Rioja is lovely. Consider adding Cordoba to your itinerary — the Mezquita is incredible. Driving is easy and pleasant, especially with Apple CarPlay or the Android equivalent.

Since you’re past vagabonds, I’ll add a plug for Extremadura. Caceres, Trujillo, Badajos, Plasencia, Jerez de los Caballeros, etc.

Rugged, beautiful country that is a lot less traveled by non-Spanish vacationers. Lots of history, great food, super cheap; even by Spanish standards

A few questions first:

Where are you flying from? You will need time to get over jetlag when you arrive in Barcelona so I would add at least an extra night in Barcelona.

What are your interests? Do you like art museums? If you like art, I would add an extra night to Madrid which has some outstanding art museums. History? Culture? Do you like to have free time to explore neighborhoods? Do you want to check sites off a list or get to know a place a little bit better?

I haven't been to Valencia so can't comment on your stop there. Haven't been to Ronda either but I would not give up Cordoba for Ronda.

Cordoba has the magnificent Mezquita along with other important sites such as the Alcazar and its beautiful gardens, the Roman Bridge, Palacio de Viana with its lovely 12 patios. The Juderia is fun to explore with its narrow lanes, white-washed buildings, patios and flower pots.

Toledo is an ancient city located on a hill and filled with Arab, Jewish and Christian monuments and history. You could visit as a day trip from Madrid, or spend a night or two.

A possible itinerary could be:
Barcelona - 5 nights (there is so much to see in Barcelona and you need time to get over jetlag)
Granada - 2 or 3 nights (fly from Barcelona to Granada)
Seville - 4 nights (Seville is beautiful with so much to see and do here; it's larger than Cordoba and Granada and deserves at least 4 nights) Train or ALSA bus to Seville
Cordoba - 2 nights (an important historical and cultural city) Train to Cordoba.
Madrid - 4 nights to include a daytrip to Toledo although I think spending a night or 2 in Toledo is preferable. Train to Madrid.

This is a total of 18 nights. If your heart is set on Ronda, you could spend 1 night in Ronda and decrease Granada to 2 nights. This is your trip; if you prefer to visit Valencia, you should do that. You don't have time for everything. But I can't imagine being in Andalusia and not seeing the Mezquita in Cordoba.

I assume you will do open-jaw or multi-city: fly into Barcelona and home from Madrid.