Here is my list of unreal engine plugins that I like
Mesh Tool by Nate Mary is a general modeling tool and the best way to do level design in the editor, though UE5's CubeGrid tool is really good too (but has fewer functions than this). You can use both together, both just output meshes. I asked Nate to add Hotspot Texturing like Source 2 has, and the madman did it. The pictures above are all of MeshTool stuff, all of it whipped up real quick.
HammUEr is a map importert that @turfster made for me like 6 years ago, which lets you import maps, models, textures, materials, etc, from the Source Engine, Quake 1-3 and Doom 3 engines, etc. It even handles pulling assets out of WADs, or exporting your textures from Unreal into Source Engine format so you can do your mapping in Hammer and import back in. It's wild and Turfster's great.
More reasons (from Joe specifically) that I wish I was on Unreal