Given your criteria, you should aim for a mix of smaller (but not nano) community fish that don’t require live or frozen food. That way you can set up an auto-feeder to dispense flakes and/or pellets while you’re away. You can put together a community by mixing/matching one species of mid/top water shoaler, one species of benthic fish, one species of “centerpiece” fish, and perhaps one species of oddball. Pick from the following lists for readily available generally undemanding and peaceful species:
Shoalers: Cherry, Checkerboard and Cuming’s Barb; Zebra and Pearl Danio; Harlequin and Blackline Rasbora; Glowlight, Neon, Black Neon, Diamond, Flame, and Head and Tail Light Tetra; Dwarf Rainbowfish; Guppy; Platy.
Benthic: Bronze, Elegant, Peppered, Leopard, and Panda Corydoras; Bulldog and Clown Pleco; Dwarf Chain Loach; Slender Buffalohead Cichlid.
Centerpiece: Kribensis, Bolivian Ram, Dwarf Flag, or Agassiz Dwarf Cichlid; Dwarf or Pearl Gourami; Betta (if chosen keep singly, pick a short-finned form, and watch to ensure other fish aren't nipping it); Angelfish (if chosen keep singly, get a domestic strain [they don’t grow as big], and don’t keep with fish the size of neon or glowlight tetras).
Oddball: Oneline Pencilfish; Glassfish; Wrestling Halfbeak; Banjo Catfish; Bristlenose Catfish; Upside-down Catfish; Kuhli Loach; African Dwarf Clawed Frog; Bamboo Shrimp. Carinotetraodon puffers and the smallest spiny eel species would work in a 20g setup also but need live or frozen food.