Can I Decrypt My Data if I Permanently Delete My Custom Key?

No.

If you have permanently deleted your custom key, the data encrypted using it cannot be decrypted. Before the scheduled deletion date of the custom key, you can cancel the scheduled deletion.

If the custom key is created using imported key material and only the key material is deleted, you can import the local backup of the key material to the custom key and reclaim the user data. If the key material is not backed up locally, user data cannot be reclaimed.