Do you really need a bath?
You can save WC space by having a wall-hung Laufen Compact Pro pan, as long as you can find a way of sorting the cistern. Geberit concealed cisterns are very slim and the Laufen WC is a very compact shape so even if you build a shallow box to contain the cistern it will take up less space than a standard close-coupled floor-standing WC. I think.
This was how I did the two WCs in our house, the frame is extremely rigid and doesn't budge, even when Granny plonks down on it:
