Hey! I’d suggest you check out this primer on how to use progressive lightmapper in URP:
Also, please take a look through one of our pinned posts which deals with lightmap artifacts, like the one’s you are seeing: [FAQ] Blocky artifacts appear when I bake a scene. How can I fix them?
Improved lightmap UV packing and unwrapping is something that high on the priority list. Currently the team is focused on bringing GPU lightmapper out of preview for 2021.2. We did introduce packing-related improvements in 2020.1. You can read more about it here: [2020.1] New Feature - Auto Pack Margin