Installation wiring is an interesting idea. Something I could consider when I move out of rented accommodation and buy a house next year.
I was surprised to find you are from Holland, from your post I half expected you to be from the USA. Their building standards are suitable for this kind of ad-hoc rewiring as they usually have wood/dry wall construction and often with underfloor access, so adding a DC circuit would be fairly easy.
In the UK most of my walls are solid concrete block. I have solid concrete floors (due to commercial premise below), so adding any wiring will involve tracking, plastic conduits, plastering, galvanised boxes etc. etc. I suppose stapling DC flex around the skirting boards is an option, but I'm not sure my landlord would approve.
I also had the issue with the Mean Well ESMPS being on back order or only available in x20s. They seem to be a US import only recently on the Euro market. I found the Recom RAC02-3.3SC and as you point out it requires you add the protection circuitry yourself. The datasheet I was looking at had the full Macoy, X, Y caps, MOV, choke, fuse etc. Seems a bit of a faff. I'll keep looking for fully contained units.
My ESP8266 board is still in the post, but does anyone know what the regulator on the dev board supports?
I went for this one as a first test:
https://www.ebay.co.uk/itm/NodeMCU-Lua-ESP8266-Development-Board-ESP12-ESP-12-with-CP2102-USB-UK-Seller/322485098225?hash=item4b159c16f1:m:mNkQGcU0dvkTkc5Hs9294LAOTA programming is something I'll look into. It's not a massive issue to go and pull a sensor and plug it into the PC though.
I am only expecting half a dozen boards to start with. Downstairs, Upstairs, boiler house (relay), outdoors (out the living room window, so only the sensors are outside) etc.
The outdoor stuff I currently only have a waterproof dallas one wire out the window, so weatherproofing isn't an issue. I intend to upgrade that to include the BME280 pressure, humidity, temp, but for that I will need a kind of Stevenson screen/house and thus I have put that off till I move house and have a shed/garage/garden to play in.
On effiency, I don't think I'm that concerned. Leakage/quiescent power and losses totalling to the order of 1 watt per device is not a big deal. Multiplying that up it's about 1Kwh every 1000 hours or 50(ish) days. So a total loss of 1 unit of electricy every 2 months per device. With 5 devices and 1 year that's a total wastage of about 30 units or £6.