Tried again with v5.4.12 final which already includes https://github.com/ezsystems/ezpublish-legacy-ee/commit/f475f5c9032eaf395adf9429927880d7357a3d9d.
Added the change to Charset in site.ini under DatabaseSettings from https://github.com/ezsystems/ezpublish-kernel/commit/b2a1ffd65a7792b541f170bb684cdc82f78ab4be#diff-079518e72cd553a190335b8aeaaf92de.
Followed the steps like in the PR.
(also tried changing charset in ezplatform.yml and i18n.ini.)
Still having issues with timeouts when publishing (although drafts are created in the process) and icons on Sub items list.
Maybe I'm missing some layer which needs to be reloaded?
Tried in UI Setup / Cache management / Clear all caches and on server side from CLI (assets:install, ezpublish:legacy:assets_install, assetic:dump, cache:clear).