The dataset you need is the PL2 or academy matches. I don't think anyone shows up to those games, but the players are vying to be full premier league professionals. So travel arrangements would be similar, physical fitness similar, tactics similar, but there would be no fans.

As for the time trend, I suspect professionalization was happening slowly, then quickly. Of course there have been fully paid footballers for a long time, but if you look at stories from the 80s and early 90s, the guys would still smoke and eat burgers. It's only in recent decades that the stops have been pulled out, and everyone started doing full on sports science to maximize every chance.

Having absolutely everyone optimized physically also means you can explore strategies that used to be impossible. If you know you're at 80% due to traveling, your gegenpress is going to be a bit less attractive.