Just tried to play SSB:U with 2 mates in an arena battle and the lag was awful. We all live within 5 miles of each other as well so it’s not like we are asking for a lagless match across the world.
Is a wired ethernet worth it for the switch?
If even one of you has a dodgy Wifi chip (if you ever had a problem with the pre-"repaired" joycons then you have this problem) then it will drop packets and cause lag for everyone in the game. You can generally tell if you have this problem by not having a full signal strength on the WiFi on a 2GHz channel with direct line of sight of the router at 8-10 meters away.
You have all changed your MTU to 1500 too right?
I have no noticeable difference between WiFi and Ethernet but I swapped my Switch rather than "repairing" the joycons so I don't have a dodgy WiFi chip. My MTU is also at 1500 and I am on a 5GHz network.