Unfortunately, like so many of these buybacks, a true checklist is never released by the company. So the checklist on TCDB is always a work in progress. For this set, it is listed as an insert set here - https://www.tcdb.com/Checklist.cfm/sid/226637/2020-Donruss---Recollection-Collection-Buyback-Autographs
As the note to the set says, “The checklist is a work in progress. Please use the inaccuracies reporting link for the set to report any missing cards. Please include the card number, name, and team in the request”. So if you have cards to add, please submit an IR to have them added to the checklist.
I have already added your Niekro card for you.