I’m replying to this for the sake of clarification.
Microsoft disabled the Windows “guest” SMB mechanisms a long time ago. However, that’s not the same thing as the guest access in TrueNAS. They are not synonymous and serve different functions.
What we were talking about here is the guest access on TrueNAS. TrueNAS uses a local user account (usually “nobody”) for any SMB requests that come in with unknown credentials. You can grant that local account access to whatever resources you want it to have access to. This is part of how SMB traditionally worked in Linux (and other Unix variants).
The Microsoft “guest” SMB feature is an actual username (and potentially, password pair) that is sent in an SMB/CIFS request. It was intended for use in Windows-only environments where all parts involved in Windows file sharing are aware of that mechanism. It was disabled for security risk issues and hasn’t been used since Windows 10 1709 back in 2017.
All this to say, that’s not the issue because it’s not used for this type of scenario anyway.