Also, go into WK settings and set the Reviews sort order to Lower Levels First, at least until you catch up. It was added a few months ago to help people work off a backlog.
sort reviews be WK or SRS level (the former in settings, the later with a script). maybe reset a few levels, so that you remove most items which you only ever got to guru1 or so, as you’ll have to relearn most of those anyway. do about 100 reviews a day. persevere.
i’ve come back from two interruptions like that (health-reasons in my case), with piles of 1600 and 1800. it might take you two months, but you’ll get there. and the day you reach zero reviews will feel grand!
edit: sorting reviews is important, otherwise, when you’ve got this many reviews, the srs breaks.