Flooring Tile - New bathroom tile - do I need floor prep?

I'm new to this whole tiling thing. I noticed some missing/cracked grout when we moved into our new house a few months ago, so I decided to regrout two bathrooms. Removing that old grout was not fun, but I put the time in, regrouted, and sealed up the first bathroom.

When I started the second bathroom, about 20 or 30 tiles came up near the bathtub, probably due to water dripping down the missing grout. At this point, I think I'm going to retile it instead.

It looks like some sort of glue was used when these tiles went in. It definitely isn't any sort of mortar. The base is a thin plywood sheet.

After reading several posts on this forum, I've learned that thinset is the way to go. I saw a bag of thinset for plywood in Home Depot last weekend, so I was thinking of using that.

Is it safe/smart to just use the thinset over this thin glue layer on the plywood? I made a few quick attempts to scrape up the glue, but it's not going anywhere. The glue is in stripes, much as you might imagine someone squeezing it out of a bottle and trying to cover a floor. I know there is a variety of other materials made for moisture locking, etc, on the market. It would be great if I didn't have to build the floor up too much and could stick with the existing single layer of wall tile, but I know I might have to pry it up if it's the best way to go.

Thanks in advance...

How thick is this ply, and if the tiles are popping up, I'd consider teh thin plywood to be no good anymore anyway. If it's less than 3/8" or had any spongy spots from water causing the veneers to come unglued form eachother, pull it out and come back with what you have for a subfloor (or with whatever treasure you find under this thin ply)

I'm not seeing any evidence of the plywood failing and it looks like it was installed pretty well, so I think I'm going to stick with it. Before I retile, though, I suppose it's wise to put a buffer layer in there. Should I be using some kind of thin cement backerboard or something? I know that isn't the right name for it, though...

I've removed all the tiles and the single layer of wall tile near the floor, purchased my cement backerboard, thinset, cutting tools, etc. Currently the toilet flange sits 3/8" above the plywood. Adding in my 1/4" backerboard, I'll be looking at 1/8" gap. I'm thinking the thinset layer between plywood and backerboard will make up for that, but assuming they don't what's the best course of action for a small gap like that? Shims? Thinset? Do I need to go buy another flange, increase the gap, and put tile under it? (I hope not)

While putting my tile underneath might sound appealing, I have 1/4" tile so it won't fit the expected 1/8" gap.

As long as the top plywood layer is not 1/4", install your cement board in a bed of thinset, and screw down, being careful to not hit any joists. Tpae and thinset teh backer seams as you set your tile. If you purchased anything premixed in a bucket, take it back, it's no good. As for the flange, if the itle end up higher than the flange, you can either use a double wax ring or get flange extenders to raise the flange height.

I've got the cementboard all trimmed out and I'll be setting it tonight with thinset. Maybe this is a dumb question, but should I wait for the thinset to dry overnight before screwing the Hardibacker down?

I would think screwing it down while the thinset is still wet would result in a tighter bond, but I don't want to screw it up if crawling on top of the cementboard causes the thinset to squeeze out.

You want to screw it down while the thinset is still wet.
Have fun
Bryan