When I was researching my 2019 trip, there was a company running a one-day bus tour to Ronda and at least a couple of the white villages. I wasn't interested in that, because I was already planning to overnight in Ronda, which is large enough that I felt a combo day trip from Seville just wasn't going to cut it. I don't blame you for not wanting to take on a marathon like that.
What I did was take a public bus (round trip) from Seville to Arcos de la Frontera. The bus drops you in the lower town, so you'll want to use some form of transportation to get to the top of the hill. I think there are buses; I'd think there would also be taxis. Arcos isn't a really tiny place, so you'll want to give yourself adequate time there. Perhaps you can make your trip align with the bus schedule by fiddling around with your lunch plans. (I've been known to skip lunch and munch on packaged peanuts so I can take the next bus out rather than waiting 3 or 4 hours for another one.)
Carmona is another good day trip you can from Seville by bus. It's attractive, with lots of white-washed houses. It doesn't have that hillside cuteness you'll find in Arcos but, on the other hand, you don't have to deal with a steep hill!
I don't think it's practical to see two of the most-often-mentioned hill towns in a single day while using public transportation, but I've read that Ecija is attractive, and it's beyond Carmona along the same highway, so those two could perhaps be paired in a single day.
Seville has two bus stations, so if you decide to take a day trip by public bus, be sure you know which station it departs from.