Community leaders @melvincarvalho and @michielbdejong voiced strong ideological concerns against CSS, including in this thread: Migrating from NSS to CSS - General Discussion - Solid Community Forum. We cannot proceed without their explicit blessing, and it’s a shame they were not tagged in this post. I believe @michielbdejong want to maintain his own fork with his company and this should be considered as a third option.
Also that thread was closed without there being a solution to migration, so it seems this is not even possible at this point at all.