Only KvD seems to ever play 150cc All Cups. Everyone else either plays time trials http://www.mariokartplayers.com/smk/ or 150cc individual tracks http://www.smkgp150cc.com

If you want to see times for the individual tracks in 150cc you're better looking at the 150cc site, it has a lot more players than speedrun.com (nearly all the good SMK players have no times on speedrun.com, so don't take any ranking you get there as a sign of how good you are).

The best 'guide' would be to learn each of the tracks individually on 150cc, watch videos of the faster track times and then when you are good, combine it all into an all cups run if that's what you want to do.

Some basic info on when I did GP150cc speedruns on my stream: https://pastebin.com/5ySMiTUu

Though it doesn't go much into the execution of the actual techniques. I guess watching runs on Youtube that karters like Neo, ScouB, Takashi and me have done would help with exploring which strategies are used (including item based ones).

Then you can always come back here with questions how a specific technique is performed. Good luck!