RE: Hidden fields in marketing forms
Hi Neil,
It seems that the issue is by design: hide field option is not available to Option set field(either radio buttons or dropdown.)
You could change its data-editorblocktype attribute to another element name, e.g: data-editorblocktype="Field-radioButtons" → data-editorblocktype="Field-checkbox".
Then "hide field" option will be available again, and from my test, marketing form can still be submitted successfully;
the only problem is that the field will be not able to set a default value by form designer tool, but we could run javascript to solve it.(After form is rendered, here is introduction about Marketing form API.)

Or we could do it without code:
From beginning we set the field a default value with data-editorblocktype="Field-radioButtons",

then change the attribute to Field-checkbox to enable hide field option.

Please wait for this issue to be fixed in future release.
Regards,
Clofly