I think you're right, that will certainly become an topic.
Before I continue with the safety discussion I wanted to check the PSU board more closely. Does it work adequately and can it deliver the promised values? What is the thermal situation under load?
I have built up the almost final circuit for testing this. Ignore the shifted component print, was my first attempt. 
Everything looks good so far. I'm feeding the whole thing with a 12V/ 2000mA wall wart switcher.
At the input of the PSU there is therefore 11.9V DC, as a load my circuit has 600mA on the heater and approx. 10mA for the B+ circuit.
This results is a heater voltage of 6.6V and a B+ of up to 297.4V! Thus the PSU successfully fulfills its target specifications of 150-280V/ 15mA and 6.3V/ 1500mA so far.
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Thermally, the whole thing is inconspicuous so far.
Let's continue, this is how the basic layout should be later on.
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The idea is to build the fx device compactly
