How to prep gypcrete bathroom subfloor that has wood around the toilet?

Question

first time bath floor tiling. The subfloor is gypcrete concrete but around the toilet it is wood. The wood is rotting so I think I will take it out. But I don't know how to proceed. So a couple of questions:

How do I know if this is not part of a bigger structure? The rotten wood can be taken out easily but the good part stick very well to the floor.

What tool can a DIY-er find to remove this without damaging the plumbing?

After removal, what should I do to repair that part of the whole floor for tiling.

If another wood piece, how do I cut and put it down? after that I imagine I will need to put in a backerboard for that portion?

Is there anything similar to concrete that can be used in this case?

EDIT: I found out that the subfloor is not concrete, but gypcrete. Oh my amateur. And I have also decided that I will hire someone to do it as I don't have the right tools and skills and time so between running around renting tools and spending time and risking doing things wrong I have decided will have to hire someone. Thanks God I didn't start working on the 2nd bathroom.

EDIT 2: More questions :/

1. Do I have to remove the vanity cabinet? I don't want to. One contractor says I need to, others don't.

2. I am given 2 options: leveler or backer/cement board.

2a. Do I have to remove the old thinset completely if I go with leveler?

2b. Can leveler be used on the wood part? I didn't know that

2c. Which option is easier if in the future I decide to replace the tile, bathtub, toilet wood subfloor, vanity cabinet or toilet?

I am more inclined to use the leveler. One contractor says I need to use Wonderboard, another says I have 2 options here. But if the thing is poured over the wood and stick to the bathtub and the cabinet, won't it be very hard to remove it in the future?

EDIT 3: I removed the flange. I poked a measuring tape into the gape and the wood is 2 inch thick and below it is 2 feet of empty space. I don't understand how this piece of wood is secured in place. How do I removed the wood without damaging the gypcrete or below structure?

I can only borrow this circular saw https://www.ryobitools.com/products/details/561

Answer 1

You've got a sticky wicket of a situation. However, being able to remove the pipe flange was a great benefit. You'll need to build-out the joist under the wall with layers of 2x6 scraps until the scraps protrude at least 3/4" beyond the wall plane for good support of the new subfloor & the ability to screw or nail.

To cut out the old subfloor, lay a bridge of plywood over the wood area, from Gypcrete to Gypcrete. Drill holes in the wood area, inside of the joist, to poke a tape measure down. Then, set your circular saw's blade depth to the measurement from the bridge's top & down to the bottom of the subfloor.

Then, you'll want to remove the nails & cut 3/4" over or on top of the joists (likely a line where the old nails were), so the new subfloor can rest on half of the old joist. Once the wood floor's out, you can use it as a template for a drop-in plywood replacement. You can do cement or cement board on top of the new subfloor patch.