I'm the developer of Ventoy (www.ventoy.net). I add support to OpenWrt in the latest release.
So we can just copy the OpenWrt img file to the USB stick and boot it through Ventoy.
But Ventoy needs the dm-mod module which is not included in the img by default.
So I have to provide a package to collect dm-mod file for all the versions. (https://github.com/ventoy/OpenWrtPlugin/releases)
So I hope dm-mod can be included into the img file in the future OpenWrt release.
Thanks for everybody.