toiled flange

Hey guys, I have a weird situation and I want to see if what I have makes sense. Old toilet flange is roughed in at around 16 inches from studs. Walls had about 1.5 to 2 inches of mud and tile so it was in reality a 14 inch rough in. There is a joist right around 10 to 12 inches. Original...

First post! I just redid the bathroom in my 1925 house, and am unsure how to secure my toilet flange. Under the tile is about 3" of dry pack deck mud, then a 4" slab of concrete. The drain is 4" PVC, cut level with the floor. I am worried the dry pack won't hold a tapcon, and I'm concerned a...