@Ejegg The portals are an independent static site, not directly based on MediaWiki. They can certainly use the new lightweight client as basis for their code and/or copy it as-is, but I believe that's effectively what they have done already. Theirs was already "lightweight" in that it had no dependencies and didn't involve fetching schemas from Meta-Wiki for debugging purposes.
I forgot that there there was a task for this. I've done this for one of your extensions at https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/FundraiserLandingPage/+/508906/, because it was blocking https://gerrit.wikimedia.org/r/508903 :)
Just to note, the new impression event pipeline (see T183978) is broken, since wgEventLoggingBaseUri was removed from global export. The solution is indeed to make the changes described in this task.