iOS/iPadOS doesn’t provide adequately granular exclusion controls over files stored locally on the device. The only way to accomplish your aim is to move the files that you don’t wish to include in your iCloud backup into an App Library (folder) - and then exclude the chosen App Library from the iCloud backup.
I have a similar need to your own - and have arranged an adequate solution for my own requirements by installing a File Management App on my iPad (specifically, File Browser - Document Manager Professional) - and created a folder structure within the File Manager’s own local App Library; this Library is explicitly excluded from the iCloud backup.
This solution has a generic system limitation (due to the iOS/iPadOS Sandboxed System Architecture) in that other Apps cannot access files within the App Library. Dependent upon you own need, this may or may not defeat the object.
File Browser - Document Manager Professional:
https://apps.apple.com/gb/app/filebrowser-document-manager/id364738545