Plan on a major redo. Our subfloor looked similar to yours. After tearing out the tile I found out that the builder held the thick rubber liner in place by nailing it down. (Insert choice words here). Was a 6x6 foot shower with 10 heads. Now it is a 3x6 insert with 2 heads. That also freed up room to put in a separate tub. Worst two months of my life. Did all of the work myself. Wife almost left me because it took so long, but she loves the tub now. And also the window she had me put in while the Sheetrock was down. It may start out as a small job, but it can grow into a monster.