@Nerces44 Like you I use the official Raspberry Pi 4 power supply (3A) because the one from Retrolag is useless in my country (France).
For booting from SSD, you have to flash the Raspberry pi 4 firmware. You can find a lot of websites talking about it.
For flashing the img file on SSD, It's the same method than with a SDCard: I use the app "Imager" from Raspberry (but you can use another tool) and I select the SSD as target.
As Recalbox will boot from the SSD instead of SDCard, you have to change an option in a file named cmdline.txt in RECALBOX partition:
That's all ! But... take care when you will upgrade Recalbox because it will replace cmdline.txt and you will have to change again the option dev=... . I've opened an issue in gitlab ( https://gitlab.com/recalbox/recalbox/-/issues/1373).