Yeah but its not like you need to do a new USB-installer every time from scratch. The USB-stick already has all the patches for every version of Macs, you just need to change the target machine you are going to use it on. OCLP will handle the rest.
In practice you just run the "OpenCore Legacy Patcher" -app and choose the correct machine model (click settings button, choose model) and then click "return" and then "Build and install OpenCore setup" and then write the setup to disk ie. to that same USB -stick. Takes about 10 seconds to change the setup on that USB-installer.