Games like Onyx
Games like Onyx include Stardew Valley, Coral Island, Sun Haven, Immortal Life, and WEBFISHING. We matched them by the tags players give Onyx on Steam (Hack and Slash, Hobby Sim, Creature Collector, Collectathon, and Fishing): closest matches first, and among equally close ones the better-known games.

Stardew Valley
Shared: Farming Sim · Life Sim · RPG

Coral Island
Shared: Farming Sim · Dating Sim · Life Sim

Sun Haven
Shared: Farming Sim · Dating Sim · Life Sim

Immortal Life
Shared: Farming Sim · Fishing · Life Sim

WEBFISHING
Shared: Fishing · Creature Collector · Collectathon

Tiny Aquarium: Social Fishkeeping
Shared: Creature Collector · Collectathon · Fishing

Diablo® IV
Shared: Hack and Slash · Action RPG · PvE

No Place Like Home
Shared: Farming Sim · Life Sim · Base Building

Path of Exile 2
Shared: Hack and Slash · Action RPG · RPG

Grim Dawn
Shared: Hack and Slash · Action RPG · RPG

Chillquarium
Shared: Creature Collector · Collectathon · Fishing

Wolcen: Lords of Mayhem
Shared: Hack and Slash · Action RPG · RPG

My Little Life
Shared: Creature Collector · Collectathon · Farming Sim

Temtem
Shared: Creature Collector · PvE · Collectathon

Last Epoch
Shared: Hack and Slash · Action RPG · RPG

Farlands
Shared: Farming Sim · Fishing · Life Sim

Path of Exile
Shared: Hack and Slash · Action RPG · RPG

Hokko Life
Shared: Farming Sim · Fishing · Life Sim

Farm Together 2
Shared: Farming Sim · Fishing · Life Sim

The Riftbreaker
Shared: Hack and Slash · Base Building · Action RPG

Clanfolk
Shared: Farming Sim · Life Sim · RPG

Research Story
Shared: Creature Collector · Farming Sim · Life Sim

Travellers Rest
Shared: Farming Sim · Life Sim · Collectathon

Littlewood
Shared: Life Sim · Collectathon · Simulation