For the record, I was finally able to disable the ctrl+shit+u key combo. What was missing aside from removing it on ibus-setup was to set GTK_IM_MODULE=ibus.
I could do it globally, but I’ve opted for now to do it only for Slack; I created a custom ~/.local/share/applications/slack.desktop file, and prepended the Exec line with env GTK_IM_MODULE=ibus