Lightmapping problems

I’m new on Unreal and I’m trying to put up some simple scene before designing a tavern. This scene consists of the default blank template from unreal with some modular floors and walls modeled by me within Blender. The problem is: I can’t get the lightmapping right.
I’ve already tried a zillion videos around the internet, but with no success so far.
Things I’ve already done:

  1. A second UV channel with a lightmap unwrapped by me, following a couple of tutorials.
  2. Islands packed by “normal direction”.
  3. Padding of 3+ pixels.
  4. Pixel snapping.
  5. Unchecked “Generate Lightmap” on import.
  6. Set the lightmap index to 1
  7. Set the resolution to 258 (floors) and 514 (walls). Watched a tutorial about the need for these 2 extra pixels. (Started with 128 and 256 respectively, but increased it later trying to solve the issue)
  8. Scene with a Lightmass Importance Volume. Tuned up some quality settings. None have solved the issues.
  9. Built the lights on preview, medium, and high quality. None have solved the issues.
  10. Using version 4.25.3

You can see my results on the images. I’m uploading images of the mesh editor with the lightmapping and the lightmap density view too.
I can live with seams between walls or floors, as they will be covered by other meshes in the end (pillars and such), but the shadow difference between panels is too much.

Any advice? I’m lost here.