Ok, after watching
this video, I've will follow your advice and changed the MOV's into TMOV's (but without indicator, because the "SP PCB" does not have an additional power source). Now I will again consider just a 3D plastic cover over the "SP PCB" which may now be enough... a rest of a risk will remain... I have not added additional fuses in series to each TMOV because the max. PV current is 10A and in case of a huge surge (lightning near by?), the fuse would anyway be too slow - I guess.
I've changed the PCB's to be more modular. The "SP PCB" could be used stand alone (without the "probe PBC" for GF/AF) and optionally cascaded side by side for 1-n strings. In stand alone usage, the back pin sockets which optionally connects to the "probe PCB" (GF/AF) are not required to mount in this case. The screw terminals will be used for PV in and PV out.
The side pin sockets are only required to mount if more than one "SP PCB" (more strings) will be used (chaining). If chained, only one module's PE screw terminal needs to be connected to PE and the other will get their PE via the pin sockets.
In case the addition (optional) GF/AF will be used, the "probe PCB" will be connected "under" (behind) the "SP PCB". In this case, the PV input will be used from the "SP PCB" screw terminal and the PV output from the "probe PCB", the not used screw terminal don't need to be mounted in this case.
A setup for 3 strings with just the SP module may have the following layout. In this case the wiring for PV in and PV out will be done from one side:
A setup for 3 strings with all modules (beside the "controller PCB") may have the following layout. No additional wires are required which makes the installation easy and everything could be placed in one case (where each SP PCB has it's own cover over the TMOV's). In this case the wiring for PV in and PV out will be done from opposite sides:
I still need to find a good way to mount and connect the central "controller PCB" to all the "probe PCB's".