Isn't 0.4 very low already? Would I find a shower designed for even lower pressure. Also I should mention in the kitchen the Tap did need replacng cos it wasn't correct bar and just came out as a trickle when hot.The manual confirms you have the version with a min pressure of 0.4 bar, so it's odds on that this is the problem.
Manual also confirms that there are inlet filters (part #3).
The filter also incorporates check valves, which aren't needed if both hot and cold supplies come from the same header tank. Removing the hot one may solve the problem, to a degree, at least.
Pretty sure its gravity fed but ill do some investigating.