Without better diagnostics, you’re stuck with old-fashioned A/B testing.
Full UI reset but only rename directories, not delete them. Verify you no longer get errors.
Reinstate WTF folder. Verify whether errors return. If so, you’ll have to either manually fix cvars or (worst case) delete WTF and rebuild UI configuration.
Reinstate one addon at a time, to uncover which are throwing errors. Update the ones that are and can be updated. Delete the ones that are erroring but can’t be updated because they’re abandoned by their authors. Those will almost certainly never get better*. If you’re lucky, you’ll find functional replacements from other authors that are being maintained and that you can learn to use.
*I suppose if you’re already good at addon development and coding, and are familiar with the release 11 API changes, you could take a turn at fixing the addon yourself, but you would be in quite the minority.