Just to check - have you tried the following? (they’re mentioned in your links but worth double checking as you don’t specifically mention them above):
- snapping LM UVs to a grid size relative to your texture size
- using padding (including at edges of of the UV space)
- using a LightMass Importance Volume
- using Production Quality (only setting that stops leakage)
Failing those I read that it’s best to separate ceiling /floor areas per room so you can segregate lighting areas. The other factor is the static lighting scale but you’ve already tried that.