(1) Build a generic and extensible UI tool which at the core is a 3D scene viewer, object outliner and an asset browser (similar to how VSCode is such a tool for 'mostly text data' for 3D scenes). Implement anything else as engine specific plugins - Blender can be that tool, but it would be important to do a complete UI overhaul to get the modelling and animation features out of the way.
(2) Integrate the editor UI right into your engine, which is quite trivial with Dear Imgui - the tricky part here is that game state data should either be organized for editing, or for runtime performance. Mixing both isn't a good idea (unless moddability is a priority).
About a decade ago I would probably have opted for option (1), nowadays I tend towards option (2).
Unity, Godot, Unreal, Defold and others kind of get away with it, by having editors that work for most game types.
But also: a tool doesn't have to cover all bases, especially a free/FOSS one.
Although come to think of it, you could probably do most of that in Blender, so maybe such an thing already exists.