I usually marked them up ahead of time, but it doesn't take very long to add a loop point on the fly. I'd usually have maybe 40 or 50 songs ready to go for a 2 hour set and play maybe a third of them? It was very very rare that I'd add in another song, usually from a request. When you're djing a lot you don't tend to change your live set that much. So doing prep work is like moving around a dozen songs between sets maybe.
There’s definitely an elitist side to the new turntable purism though and I think DJ-ing with software is much more practical and accessible for most. Although I think it makes sense to try out turntables at some point just to understand where it comes from.
When you talk about mixing in Ableton, the one thing I wonder about is the live aspect. In most DJ-software it’s super easy to set cue points and loops and do remixes on the fly. As far as I understand, with Ableton you’d have to prepare your track properly by cutting it up in ‘clips’ before-hand. Or does that become super fast with practice?