Yeah, meshes with lots of individual floating parts are not ideal, and that’s causing some of your issues (not sure if it’s the only problem) Unwrapping UVs for Lightmaps | Unreal Engine Documentation
For something that is baked, you ideally want as few UV islands and floating parts as possible. I’m not saying it needs to be water tight, or have no floating details, but instead of having dozens of parts, try to keep it under a dozen.