On the Psychic note I'd add Drowzee/Hypno if you were lucky enough to get it. There also is Spoink/Grumpig that can be very solid. Alolan Raichu is already available (use thunder stone while on Apophyll). Not a psychic type, but Ariados learns Phychic around lvl 40, resists poison attacks and has some very good utility with Sticky Web and will be very useful for the Dark type gym later.
Ground types are rather common at this point already. Just mind that Earthquake or Bulldoze are MUCH weaker on that terrain. However moves like Earth power or Dig are fully effective, so Dugtrio/Camerupt/Nidoking are your best bet.
For steel Types, you also have Forretress the ultimate wall (except for fire)! Use headbutt on the trees near Reborn center to get a Pineco. You can get Klink in the railway underground as well.
You might also want to try Pledge moves on your pokemons. If the terrain sucks for your team, just change it!