I would like to nominate @Dreamy_Jazz for +2 in mediawiki/*. Dreamy is one of the CheckUser maintainers (+2 received in T311331) and also admin, checkuser and oversigher in enwiki. As such, he has worked a lot on the extension, which he uses a lot himself.
He has started major refactorings in CheckUser, among other things to fix some security issue, see for this T324907 and tasktree. In the patches to this refactoring, he has already strongly demonstrated that he has a broad understanding of the mediawiki codebase and the wmf infrastructure.
Since he has +2 he very actively reviews almost all patches submitted to CheckUser, in these reviews he works very carefully and pays attention to details in a way that some others (including me) do not (e.g. by testing DB related patches on different rdbms' and checking for weird edge cases). Among other things, he reviewed most of the patches for the checkuser actor/comment migration (T233004), which went largely without incident. I was impressed by this accuracy from the beginning, for example, he has also set up an extra "beta cluster" type wiki for checkuser (https://checkuser-beta-wiki.wmcloud.org) since checkuser is not installed in the classic beta cluster.
He also started writing patches for other extensions (in T327431 there is a +2 request for ContactPages).
I think giving Dreamy +2 rights would be beneficial and look forward to even more contributions from them :)