I came up with some own rules that made the chances of winning equal for all players, independent of their respective skills.

Common Rules

• We've been more than 8 players so top players were exchanged after each match.
• Each match had a 5 minute time limit rather than stocks, so nobody had to wait for the end of a match.
• Additionally we've played different rounds, with slightly different rules each.

The warmup round, 5 Matches with these rules:

• No items
• Free character pick
• Big Battlefield only

The rules of this round were the same as in round 1, but the players had to choose fighters, who fit into a certain theme. We've played 2 Matches for each theme:

• My Fair Lady - only female fighters were allowed to be picked, e.g. Samus, Peach, female Corrin
• I've got something on my head - fighters with headpieces had to be picked, e.g. Ness, King K. Rool
• Cute and cuddly - only fighters, who were exceptionally cute were allowed to be picked, e.g. Pichu, Yigglypuff
• Furious Fatsos - Big, heavy fighters had to be picked, e.g. Donkey Kong, King Dedede
• The F are you!? - the players had to pick characters that are 'indefinable' like e.g. Inkling, R.O.B., or Pac-Man

This one was the most chaotic round. Like in the previous round, there were certain topics, and we made 2 matches for each. The common rules here were random stage, no stage morph, certain character picks and one kind of item, that spawned a lot.

• Mr. Saturn Invasion - only Mr. Saturn was spawned and players could either choose Ness or Lucas
• Mega Ultra Chicken - Chickens and Bees were spawned a lot, and only Link, young Link, or Toon Link were allowed
• Bombchu Mayem - Bombchus spawned everywhere, but free character choice
• Warp Star Smash - everyone had to pick Kirby and Warp Stars appeared all over the place
• Falcon Football - everyone was playing Cpt. Falcon, we played on the Super Mario Bros. Stage, and footballs spawned everywhere

Things calmed down a bit here. The rules where pretty much the same as in round 2, but the players had to form teams of 2. The teams then had to fit into the following topics:

• Free character choice
• Teams by series, e.g. Team Hyrule, Team Mushroom Kingdom, Team Pokémon
• Teams consisted of one character, e.g. Team Kirby, Team Fox, Team Ganondorf
• Every Player picks a character for somebody else, e.g. I as PlayerX tell PlayerY she should pick Olimar
• Everyone has to pick random fighters

I couldn't resist to include a round of a classic tournament. The rules were simple: 3 stock, no stage hazards, no stage morph, no items, one on on. Whoever wins the tournament wins the round (but not the entire casual tournament).

It did take a fair amount of time to put all the rules into the Switch, of course. The most complicated thing was the random stage choice, though.
I'm under the impression the Smash Ultimate stages became the most convoluted, unclear and confusing of all smash games. I've tested every stage with 7 NPCs and decided, which to admit into the tournament. If I thought there was enough space for 8 players, and if I thought I always knew where I was, played an important role in this process. Admittedly those are very subjective measurements.
In the end I had a bunch of stages I found acceptable for the tournament, though. To modulate them for each of the above rules, was a total hassle (as if playing each and every stage once didn't take up enough time), but I managed.