It’s very strange! I find an un-clickable checkbox very disconcerting, especially when it deals with the filesystem. What’s even stranger is the fact that you just did a clean installation, because this sounds like a problem somewhere in the operating system. It’s hard to know if it’s a minor problem or not (that is, will this problem only affect file sharing, or is something more damaging going on?).
In case you didn’t come across this Security/Privacy suggestion in System Preferences:
The process smbd is related to SMB file transfers, and thus file sharing. It’s worth taking a look at, though if smbd doesn’t appear in full disk access, I have no idea how to add it! But if it is there and turned on, I’d try toggling it off, waiting for the process to stop, and then reenabling it.
Another thing — did you use Migration Assistant to move data to this Mac after it was updated? If so, that could have brought a damaged file or could have damaged something in the process. (Note: it shouldn’t have, but clearly something went wrong.)
You could create a new user and see if you can enable file sharing for that user. That should tell you if the problem is at the operating system level or if it’s unique to the account.
If all accounts are affected, you could try reinstalling the operating system. I’m pretty sure you can just reinstall right on top of the current installation without removing your data. I’d try that, first.
If that doesn’t fix it, then I’d wipe the disk and reinstall the operating system. Then bring your data over without using Migration assistant.
If the new account is not affected by this bug, then I’d move my data to the new account. Don’t use Migration Assistant, just in case. Instead, just move the data and reinstall apps. You won’t easily be able to copy data from one account to another — its much easier and more reliable to copy the data to an external drive from the original account, and then from the external drive to the new account. (Doing so avoids problems with permissions, as permissions are not (usually) enforced on external drives.)