What needs bearing in mind is that auto gain simply sets the peaks to a certain level. It doesn’t alter the dynamic range of the track, so the issue with playing a modern track alongside an old track will remain.
Having to pre listen and check levels for every song beforehand - yes, that’s precisely what DJs have done for many decades, and it’s perfectly doable. DJ equipment has gain knobs for that exact purpose.
“Live DJs have to rely heavily on good autogain”? IMO that’s not the case.
To my knowledge there’s not a single piece of DJ hardware on the market currently that has auto gain. Anyone DJing with standalone equipment will not (cannot) be using auto gain. It’s a software option, and it’s a convenience, not a necessity.
If you find manually adjusting levels too much hard work, then use of a compressor as suggested is the way to go.
