To answer your question about using the fixed shower head and rain head at the same time - we have them on the same valve with a 3-way diverter, so it can be all shower head, 1/2 and 1/2, or all rain head, by turning the diverter. I warm the shower up on the 1/2 and 1/2 setting so that there is warm water in the rain head pipe, then I tend to use the shower head alone, then the rain head alone. I tend to not use the 1/2 and 1/2 setting once I get in. Warming it up on the 1/2 and 1/2 setting eliminates the cold blast when you change over to the rain head. We placed the rain head in the ceiling in a way that it would only be a tiny step from the regular shower head, and not so close to the door/curb and bench. I like how it worked out bc I can put a towel on the bench and it doesn't get wet.

In our 4 x 6 shower, there's the regular shower head, the rain head approx 2' from that wall (so not centered), teak (move-able) bench on the opposite end of the regular shower head, with hand-held on the side wall. When sitting, the hand-held is at the right spot for easy reach and for turning the valve. The rain head does not make us feel like we are drowning, we really like it. But some people don't like them.

You could do a fixed shower head, and a hand-held on a slide bar (to take the place of the other fixed shower head, and the hand-held), plus the rain head. That would give you all the things you were listing above. If you want only 1 value, you could get a 6-way diverter so that you can divert and have 2 fixtures going half way at a time. I don't have a thermostatic and don't really understand what those are. We have 2 pressure-balance valves - the rain and regular shower head are on 1 value with a 3-way diverter, and the hand held is on it's own valve. In some areas there are rules against using more than one fixture at a time unless you are diverting it, so that the water-usage stays the same.

Depending on where you live, the large shower could be quite cold unless you are talking about a steam shower or enclosing it with glass and no open top. Ours is not a steam shower, but it does not have an open top, so it holds in the steam and heat. We are in the Gulf South, but it would still be cold for me with an open top because down here we have the AC on 10 months of the year.