I was disappointed in Everybody 1-2-Switch because there was no option for single player mode.
Mario kart?
I know Super Mario Party allows for single player. Not positive on Mario Party Superstars since I’ve never tried it single player but I would be quite surprised if single player isn’t supported on that. That said, my go-to party game is actually Smash Bros. Ultimate and that definitely supports single player.
Mario Party Superstars has a single player mode too as well as 4 person co-op both online and offline.
WarioWare.
Lovers in a strange space time
Stardew Valley, most Lego games, hmmm… Luigi’s mansion 2, Super Mario 3D world, Super Mary Odyssey, Pikmin 3, KeyWe and maybe Children of Mortia? We play a lot of cozy games and those are our favorites