"Do I file as a Simple Trust? Grantor Trust doesn’t seem to fit."
I am very sorry for your loss. Upon your father's death, the trust became irrevocable and generally cannot file as a grantor trust; as trustee, you would file a Form 1041 (if required) as either a simple or complex trust.
Whether the trust is simple or complex depends upon the terms in the trust instrument itself.
If the trust is required to distribute all income currently, does not provide that any amounts are to be
paid, permanently set aside, or used for charitable purposes, and does not distribute amounts allocated to the corpus of the trust, then the trust qualifies as a simple trust. Otherwise, the trust is a complex trust.
See https://www.irs.gov/instructions/i1041#idm139674712546640
