I want to know what I need to look for, prep for, and consider alternatives for, with my situation.

I have confidence that I can do this on my own and with my DH's brawn & brains.

Here is under my bath. the joists are 8x2 (but not quite, is that called nominal?). Unfortunately the joist are 21" on center. The diagonal flooring boards are 6" or so in width, and I presume 3/4"-1" thick as I estimate the thickness looking at a knot hole. These boards have a slight gap between them:

Top side - with the toilet removed it appears that there is 3/4" fir tongue & groove flooring over the diagonal boards. Then over the T&G there is an old linoleum floor. I don't want to add more than 1-1/2" to the height of my floor from the T&G fir flooring, preferably 1".

1) Will this floor situation be stiff enough for tile, as is, with the subfloor and fir flooring?

2) Is it recommended that 4 more joist go in between the 5 that are there. Currently there is 1 at each edge and 3 filling the middle, at 21" on center.

3) Do I add plywood atop the T&G to add more rigidity?

4) What is the depth of a slate tile + prep + electric underfloor heating?

5) When does the floor go in place? I will be relocating toilet, sink, putting in new tub, taking out plaster to insulate exterior walls, new electrical, etc.

Thanks for your consideration.

The spacing of the joist is 21" OC. The length of the joist are about 10', free span, but possibly 12' total taking into account the ends that sit on the plate.

I had meant to ask on my post if there would be a tile size limitation such as 12x12 or 16x16. I was thinking of doing a small Versailles pattern where the largest is probably around 16x16.

As I mentioned, this part of the house is joisted differently than the rest of the house, I think they where saving some lumber;) My DIA (dear inlaw ancestors) had this bathroom addition put on over the crawl space after they moved this small house from town and onto the farm. The rest of the house is sitting over a regular basement. My late DFIL and his sister hand dug the basement when they were teens. They used the horse team and a slip. DH remembers his dad talking about how hard the soil was. His DD said when the slip hit the hard soil and bucked, he'd end up on the backs of the horses. He must have been standing on the slip.