Thanks for reaching out. To clarify, when the channel is moved to Brand Account, you'll lose the custom URL. Have you tried changing the URL by following the steps mentioned in this Help Center resource: https://yt.be/help/KHjM ? Let us know.
Thanks for trying those steps. Please send a feedback report directly from YouTube: https://yt.be/help/a4gp . While every individual report may not get a direct response back, we do review everything and use the details to investigate and fix issues.