"Should I file original grantor trust return in 2020, than another return for 2020 as a simple trust after grantor's death"

What would you put on these returns? The property is not income producing.

Definitely do not file a 1041 for the period of time in 2020 Grantor was alive.

There is an election that can be made to treat the trust as part of the estate. That may simplify the filings and get you a fiscal year end. See Form 8855.