You need the 90mm void to accommodate the drain and trap.
If you are determined to have a wet room you could make a false ceiling in the room below but it would look a bit odd. Better to fit a slimline tray with a side panel , you could fit a moving return to it to stop splash. I can’t imagine there are many houses without sufficient depth between the floor and ceiling to accommodate a top access shower waste.It would also be a cheaper option than tanking.

你好,我认为你的意思是湿的房间——而不是走路in shower (which can be a tray?). Firstly, I would take the floor up to view the depth and run of the joists. Depth wise it is very unusual (I've NEVER experienced it!) to not have enough depth to get the shower trap between joists. The usual problem is the placement of the waste hole in the wet room tray and the run from the joists in order to get the 40mm waste pipe out through an external wall into a soil pipe or into an internal soil/service pipe. Once you have determined you can do this then you need to look at wet room trays (Jackoboard, etc) where the waste hole is positioned so it can sit between your floor joists. You will also need to create a sub-floor between the joists to accommodate the wet room tray being flush with your flooring. I would suggest the whole flooring coming up and doing this and replacing the floor outside of the wet room tray with P5 flooring topped with hardiebacker 6mm cement board if you are tiling and if you are not tiling outside of the wet room tray area and are using LVT pr LVP then no need for cement boards. I have done plenty of upstairs wet rooms and they can be done without having to incorporate a plinth. Also, buy the best waste and trap on the market and ask to see previous wet room jobs from your installer as I have seen plenty of bad wet room jobs. Fitting a wet room and fitting a shower tray are two different skills. Best of luck

I think a linear drain might be a better option as they work and look great

If your floor joists are only 3 x 2 then you will not be able to cut into them to put a shower in the floor, you will either have to build floor up or you need to reinforce the current joists to hold the tray. You could buy some steel plate and clamp joists between them to strengthen the floor. If it's a slimline tray you may only need to cut the sub floor out and rest the tray on extra bearers.

And tank the shower room completely, stop trying to save money, it will only cost you in the end.