Back doors in southern or central Spain?

Adjacent to Ronda you have three large parks, endless options. My plan when I get back to the region is to park in Gaucin and hike.
Being a big fan of wetlands, I'd also like to get to Donana one day.
Are you driving this entire route? A rough sketch of the plan will help others offer advice. Also, what month? Suggestions for August will greatly differ from those in October.
If you are driving from Sevilla to Oviedo, for example, you could spend months in Extremadura--one of the most beautiful parts of Spain that I have seen!

See--details matter! I was envisioning a trip taking place over many weeks, and you have five or six nights to play with. Where do you need to be for departure? With this framework in mind, I would consider adding a night to Granada, then rent a car and enjoy the parks near Ronda. I have only done a day hike via public transport out of Ronda (https://www.andalucia.com/rural/walking/railway.htm), but I have heard great things about Grazalema.
Or, if Extremadura captures your imagination, you can take a train to Caceres from Sevilla and rent a car as needed. The region along the Portuguese border is exceptionally pretty. Hiking with no others!

I'll mention that much of the back doors you are looking for are not so efficiently connected by public transportation. So a car in almost required for many of them.

First back door I'll recommend (no car necessary) is spending a night or two in Cordoba, my favorite city in Andalucía. The true charm of Cordoba comes out in the golden hours of the afternoon and evening, when all the day trippers from Sevilla are gone, the locals come out, and you practically have the old town to yourself. You can then daytrip to the nearby Medina Azahara, the ruins of fortified palace-capital of Caliphate of Al-Andalus (aka the original Alhambra).

Next suggestion is rent a car from Sevilla and drive to the quaint town of El Rocío for a few days, located in the heart of the Doñana National Park, just southwest of Sevilla. El Rocío is a traditional white-washed Andalucian cowboy town, with its old taverns and dusty streets the town feels like something from the wild west. El Rocío makes for a great base to explore and hike around the Doñana National Park, a wild and beautiful wetlands of the Guadalquivir river, and UNESCO World Heritage Site. Virtually unknown to foreign tourists, it's an authentic slice of the little known Andalucian cowboy culture.

Finally from Sevilla you can also rent a car and explore the southern half of the Extremadura region for a few days. The vastly underrated Extremadura region is a land of time warped Crusader Castles, Moorish Strongholds, Royal Monasteries, and impressive Roman ruins. This is the region which the Holy Roman Emperor Charles the V retired to, it's also home to the best quality of Jamon Iberico in Spain. I'd recommend charming small towns like Monesterio (for Jamon), Jerez de los Caballeros, Zafra. Extremadura should be lovely that time of year, with groves of oak and fields of wild flowers.

you can read more here: https://www.lonelyplanet.com/articles/unexplored-spain-roadtripping-around-extremadura

The first things I'd do would be include Cordoba and add a night in Seville.

Ubeda and Baeza, in northern Andalucia, see comparatively few foreign tourists. The architecture is very different from what you'll see in Cordoba and southern Andalucia.

I agree that Extremadura is very interesting and would recommend Caceres as a good base, with a side trip to Trujillo. There are some Roman ruins and a good archaeological museum in Merida if that sort of thing holds particular interest to you. Merida is otherwise not especially interesting.

Yep. Spain is a very diverse place, and it's a very large country.

We like visiting Barcelona, but never cared for other Mediterranean beach cities in Spain. Last trip, we flew into Madrid, took a bus down to Granada and flew over to Barcelona. Then we took the fast train to Paris.

We love the interior of the country, however. We've got to visit the northern part of Spain next.

I also thought you were planning a much longer trip, 3-4 weeks. The glaring omission is Cordoba so add it for at least two nights. La Mesquita is a treasure! Sevilla needs more time, five days.
You mention Barcelona with a day trip to Dalí. Do you mean to Figueres to visit the Dalí Museum or perhaps Cadaques where he lived? Both are very worthwhile but not “ Back Doors.” These locations are very worthwhile to visit but they are not off the beaten path which you seem to want.
You have the framework of a very good trip but your desire for smaller back doors and natural landscapes are not here in your itinerary yet and it doesn’t seem that you have enough time to add them even if other posters recommend some.