Dear respected dizmed,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, when we create a board view in SharePoint online List the "unassigned bucket/; in the first place we cannot remove/edit the board view.
Regarding to, "adjust the column width of the cards" based on my test unfortunately, the column formatting JSON approach does not work directly in the board view of a SharePoint list. The column width in the board view is determined by the layout and design of the board view itself, and there is no direct control over individual column widths. We feel your pain due to this limitation and we’re really feel sorry for it. However, we appreciate your understanding. I do understand the inconvenience and apologize for it.
Here you can see if I add JSON code and it won't affect in a board view.
As a workaround, you may use Microsoft Power APPs solution, e.g., in Microsoft Power APPs you can create a card view and customize board view and then embed into SharePoint online site page.
Appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Waqas Muhammad