Hi all. Newb here. I've done some remodelling and construction, but have run into a doosy when I bought a rental home. The upstairs bath (small, about 3x6) had spongy floors. Upon tear-out I found a center joist cut lengthways AND in half for shower plumbing install (Bastards!!). So, I joined the joist with 2x4's screwed to each side, then screwed more 2x4's to those to support the plank flooring. I screwed planks over that, but they ended up being fairly short pieces - not ideal. Over that I put 1/2 ply (again, smaller pieces due to shape of room), then Durock.

Issue I have now is that the Durock, although "fairly" solid is not 100% solid and not 100% level. Should I use a self leveller, then tile it - or should I use a thinset and a final layer of Durock befoe tiling, or ??

Thanks in advance for your help!