My understanding is, the presidential pardon power is based on this passage of the US constitution:
The President ... shall have Power to grant Reprieves and Pardons for Offenses against the United States, except in Cases of impeachment.
I've read that one issue* with the uncertainty about the legality/validity of presidential self pardons is the word "grant" wasn't really used in the context of "grant myself" at the time the constitution was written.
But the pardon power can also apply to entire groups of people. And the president can belong to a group. Based on this, would a presidential pardon become more likely to be legal/valid if it applied to a group of people that included the president, instead of just to the individual? Even if the group was very small and the attempt rather transparent (e.g. "the Trump family", or "current and former Presidents of the United States of America")?
*I'm aware that this is not the only issue, and a possible answer is that the "grant" issue is dwarfed by more relevant legal arguments.