Andalusia: Bus/Train tickets timeline

Arcos has no train station, so you can stop looking for train tickets to get you there.

Ronda has a train station, but the rail options from Seville are not very good. The route would be round-about and therefore both slower than you'd expect and unnecessarily costly (you have to pay for all those miles you'd rather avoid). I don't know whether Renfe is having a technical problem that prevents it from showing the 1- or 2-change routing you'd need to follow between Seville and Ronda or it's a conscious decision not to sell that route. You can see the rail options on the German rail website, but not buy tickets there: https://int.bahn.de/en. Use Sevilla-Santa Justa as your origin.

A bus would probably be better. I think you're just too early to see tickets for May. Yes, you might save a bit of money on some routes by buying a bus ticket early, but I think bus tickets don't go on sale as far in advance as train tickets. You can get an idea of how far in advance bus tickets go on sale by checking the Damas and Avanza websites to see what is the latest date being sold right now.

We traveled by bus from Malaga to Ronda to Seville in October. I just checked and we bought tickets 30 days in advance online. Knowing myself that was probably it the first day they went on sale.