Hi! This is the way it worked for me. I'm posting it step by step so it will be clear: In order to install windows 7 on a Ryzen cpu (or Intel i9 and above) you need a motherboard that has a PS/2 connector and chipset drivers from your motherboard manufacturer that support windows 7. If one of those two, or both are missing, you cannot install windows 7. 1) Make a bootable usb stick with windows 7. 2) Go to the Gigabyte's web site and download windows image tool (at this moment it's version B18.0213.1). Run windows image tool and select "NONE - Add usb drivers". If you have an SSD hard disk, tick add NVMe drivers to an offline windows 7 image. If the application after finishes, displays a message that the operation failed, just ignore and continue. 3) copy the windows 7 chipset drivers to a cd (if you have a cd-rom on the installation machine), otherwise to a sata hdd. 4) Go to BIOS on the machine you want to install windows and set all usb mode to legacy. Also set CSM enabled if available. 5) Connect a PS/2 keyboard and install windows 7. Be careful during boot options, not to select usb with uefi support if offered. During the installation you will have mouse. 6) After the installation is finished and the system logins to windows, there will be no mouse and no usb support at all. 7) If you don't have a cd-rom (so you can just insert the cd with the chipset drivers), shut down the pc and connect your hhd with the chipset drivers (as secondary sata). 8) Navigate with your keyboard from windows to the hdd partition with the chipset drivers and install them (tip: use the windows button on your keyboard) 9) Restart your pc and now it will boot with usb!
Did you try using windows 10 boot.wim and use dism or dism++ to apply the image from win7 install.wim. Integrate any needed drivers into the applied image using win10 dism or dism++ before booting into it.