Hello @HARUKI TUN
Thank you for posting this concern on this community space.
I have read your case scenario description and I wonder if the following steps stated were reviewed by you at some point:
https://learn.microsoft.com/en-us/windows-server/remote/remote-desktop-services/migrate-rds-cals
https://learn.microsoft.com/en-us/windows-server/remote/remote-desktop-services/upgrade-to-rdsh
I hope that can be useful for you.
Looking forward to your feedback,
Cheers,
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.