Looks like this has come and gone, but for those in the future thinking along the same lines, the percentage move, I think, would be to skip the coast and take a couple of the rails to trails Vias Verdes routes that run on the other side of the mountains.
Of the cities they want to go to, Gibraltar, Marbella, Benidorm and Malaga aren't that great - tatty in places, overrun with drunken tourists and pretty much paved over. The Costa del Sol is well set up for boozy package tour holidays, but it's not the sweet spot for a bike tour. An alternate route would start in Cadiz, jog up to Jerez, a wonderful town to visit, and then head for the Via Verde de la Sierra. There's a moderate hill getting up to the start, but doable, and the rail to trails Via Verde is pretty flat as it was laid out for trains. There are backroad connections from the end of the Sierra Vias Verde to the Via Verde that runs from Puente Genil to Jaen. Once you are on that Via Verde, which runs car free for 130 km or so, it goes past some really sweet White Villages up in the hills, but again rolling on level grades laid out for the railroads. From Jaen, which has some sights, it's a short ride to Granada, another magical town, from which you can ride down to the coast at Motril or over to Almeria.
It's easy to roll bikes right on on the local, non-express trains in Spain, so if you want to skip some hills or some areas you do not find interesting (e.g., Granada to the coast at Motril or Malaga), just hop on the slow, cheap train.
This route would also be, I think, less hot as you do get a bit of elevation, but even in the hills Andalucia in the summer can be rudely warm.
I rode a tiny portion of this with my daughter last winter, and have a write up at CrazyGuy that also provides links to some of the relevant websites.
https://www.crazyguyonabike.com/doc/?o=1&doc_id=11781&v=5R
