It's a good development that we have more merge requests coming in and things are getting looked at. As things pick up, we need to be able to quickly identify which requests are waiting on review, where we're waiting on the review comments to be addressed.
I guess we can keep switching the WIP: flag on the request for that one and let authors remove it when ready again.
Similar when a different reviewer is needed (e.g. when I've added substantial code to an existing request) or a configure update or a backport to an older release should be done post-merge, labels might or might not be the best fit, there's also way to request review from a particular person.
BTW non-project members are invited to help with review and testing, it's a great way to get familiar with the codebase and helps the project move faster and at higher quality.