Torsten, what I did was get the following from (US) Amazon:
https://www.amazon.com/dp/B0D5B82V6K
This connects to slave on-off control input (J5). When the switch is in the off position, a voltage here turns the unit on, and no voltage turns the unit off.
I then used a USB brick on a switched outlet to turn it off overnight for charging, and have it on during the day
Other elements for my build for the combo of the Ian Canada mini battery + Ian Canada conditioner + the WR902AC (since tracking these things down for a first build is a pain, hopefully there are equivalents from local options in Europe)
https://www.amazon.com/dp/B07PKKY8BX?ref=ppx_yo2ov_dt_b_fed_asin_title
https://www.amazon.com/dp/B0F9TP2GZ6?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1
https://www.amazon.com/dp/B0CZ6WKTN5?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1
https://www.amazon.com/dp/B09XWS2FTM?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1
https://www.amazon.com/dp/B0CCRXF1N7?ref=ppx_yo2ov_dt_b_fed_asin_title
For the enclosure, I have one USB C input for charging, one switched USB C input for turning on and off, and ethernet output. I purchased standoff screws from Ian for the mini battery PCB, conditioner PCB, and the WR902 (I drilled mounting holes in the bottom of the case when I had the PCB out)
I am currently waiting on delivery of parts to build a v3 throttle cable. Looking forward to hearing what impact that has with the WR902
As an aside, note that Ian let me know that a new Pro version of the LiFEPO4 mini battery adapter is getting close to release, that support faster USB charging and have some other improvements