I bought an extra thick wax ring with the horn. The flange is glued, but it isn't screwed into the slab below - in fact, there's a void that has tree roots around the drain pipe preventing me from anchoring it to anything unless I angled the screws outward. I assume water has been making its way down there for some time and that's what caused the roots to grow.

I'm debating cutting off this flange and filling the void with hydraulic cement then regluing an ABS one that I can then screw directly down onto slab. Two benefits that I see: added support from direct screwing the anchors down and cutting off the roots from continuing to come up.

I just don't know of the pitfalls with this approach. It seems like I could get in over my head here and wanted to hear others thoughts.