Glad you got it figured out!
I would hazard a guess from what you’ve found that the relays are pulling some power on boot and lowering the voltage enough for the esp chip not to start? Just my guess - but that could explain it.
Also with a relay on GPIO15 - this could interfere with boot if not set correctly - just food for thought.
Cheers!
DeadEnd
Yep sounds pretty logical 
very good point about that GPIO !!
For the power on button I tested the board with my multimeter and there is no direct link between button and any GPI. It’s connected to GND and I get a 900Ohm resistance between VCC of the ESP module and the button. Any ideas how it works ?
Just taking a quick look at the traces on the board - I don’t think the power button interacts with the ESP at all. I think it is just a literal dumb power button. That would make sense since there are so many different components (what crazy button clicking would be needed to control all 4 relays!).
Reading on other similar products, I think this is correct - but again, someone who is more knowledgeable can correct me. I think the button is more of a device on/switch like you would see on a surge suppressor - it kills all power to the device.
Of course you can test this… does the ESP8266 go offline when you press the power button? If so I would say it is hardware kill switch - not programmable button.
Cheers!
DeadEnd
It’s not a killing power button as since I have reprogrammed device with ESPHome this button is not doing anything at all 
Side note: it’s a push button not a maintained one, and low voltage/power one !
Did you try GPIO13?
I’m not sure what you used to identify the GPIO’s previously - I actually just did the below procedure last night with a single socket plug.
Using the tasmota software I assigned a relay to the GPIO’s I wanted to check. After it restarts, you can then flip each in the GUI and see what GPIO controls each relay. After this do the same with switch/button assignments. This lets you identify what GPIO controls the switch.
If you haven’t already some something like this you could try the second half - it would require you to flash the device again, but could help you identify what GPIO is connected to the switch.
Make sure to disconnect Main power when flashing!
Cheers!
DeadEnd
Vincen ?
You succeed ? or not ?
If you have some trouble with this powerstrip
I can help you
I recovered from bricked powerstrip to Awesome ESPHOME node.
I wish you can use this powerstrip without trouble.
I came across this topic searching for a solution for my problem with a device that looks almost the same as this one and thought of sharing my experience with changing firmware in it because I thought I broke mine also this may not be the same situation as mine but maybe this will save someone some head scratching. My device is called SL-PS26.
I did the steps for the OTA detailed in the SuperHouseTV video:
There is a step in the beginning of the conversion when the original firmware is downloaded. It failed for me - timedout. I wanted to be on the safe side. Restarted the device but it did not go back to the normal operation. No button pressing helped and the device was missing from the tuya app.
After some searching and head scratching it turned out - or at least this is my understanding - the device was stuck in the “update firmware mode”. It would still connect to the “vtrust-flash” network.
I could connect to it through address 10.42.42.42 and return it to operation by going to 10.42.42.42/undo.
Then I tried the OTA again and again failing with the download of the firmware. There is an option to try to download the firmware by going to 10.42.42.42/backup. The first try failed but when I tried it again after a minute or so it downloaded ok through a browser.
So the final solution was. There is a step in the tuya-convert when one is asked just before firmware download to connect another device to the vtrust-flash network. During this step I needed to connect to 10.42.42.42/backup twice from another device. Once for download to fail and once for download to go ok. Only then the download in the tuya-convert would succeed and progress to flashing firmware.
Tasmota Tempalte that works for me:
{“NAME”:“SL-PS26”,“GPIO”:[52,0,53,0,25,22,0,0,24,17,23,21,0],“FLAG”:0,“BASE”:18}