When I designed the alpha board, I thought that most SIL-PCB relays and SSRs had a very similar footprint. But when I went to buy an SSR I found that by far the most common cheap SSR has the same layout but in a larger footprint. So for the alpha board test, I've put it on wires and tucked it underneath the PCB.
Having killed a couple of the small BS170 MOSFETs by running without a gate resistor (which the original design had) I bought some 2N7000 MOSFETs which are cheaper and lower rated but should be good enough. Annoyingly their TO-92 pinout is not the same but at least it's just rotated 180 degrees, so still fits fine.
I also reinstated the MOSFET's gate resistor with a 470 ohm to match the LED resistor, thus reducing variety & assembly costs.
A handy aspect of the flexible configuration by solder-jumper & easy-access pin headers means that with a single jumper wire from GPIO0 to GPIO2, I can test the transistor, MOSFET & output device using just the on-board trigger button. No need to have an ESP-01 installed!
I then put this board into the original project box, replacing the separate 555 monostable & SSR boards. All works fine.
The latest PCB design includes these and other changes and now that this last test is complete I'll be sending off for the new PCBs soon. I'll probably get them assembled with the essential/cheap components as SMD, leaving regulator, header pins, sockets & screw terminals to the user. And of course leaving the final output device free for the users choice.