Godot, it's the most mature of the bunch. It's a little different than Unity, but it's definitely very user friendly, really powerful and has an active community.

Godot is probably the best choice for open source game engines. Its got funding and full time developers working on it.

Stride3D is probably the closest open source clone of Unity. It was developed by Silicon Studio as a commercial game engine but they eventually stopped and open sourced it. Its got a ton of modern features including vulkan and direct x 12 support. It has an active community too, but no full time staff making new features.

Godot Engine is a nice alternative.

There is, unreal besides being a product has its source available and Godot focus on the same niche most of unity games were. But the problem never was the lack of replacement, the problem is, a game with years of development on unity whould not easily switch to any alternative, they have assets from unity store, scripts made for unity, UIs using unity specific stuffs, even network protocols could be bounded with unity. Change this is an herculean task and most of the games are in barely maintenance mode, imagine a full rework. So these games should be pulled of the market and thrown in the garbage to avoid new installations.