Hrm. I've always read the recommendations from freenas.org and USB sticks still seem to be mentioned as perfectly viable solutions for the OS device for 11.2 [1]. Is there different documentation elsewhere that I should be following?USB sticks are not recommended any more, you are reading outdated recommendations. You should not use a USB stick for FreeNAS, they are far too unreliable. See if you have an empty M.2 slot or two on your motherboard, SATA DOM, or do some expansion with a PCie slot or something. There are plenty of creative ways to get additional slots available.
[1] https://www.ixsystems.com/documentation/freenas/11.2/intro.html#the-operating-system-device