Trump's corrupt pardons can be the basis of obstruction of justice charges
and seemingly other criminal charges, too.
It seems to me that many people are confusing the law giving power to take some action with being able to take that action totally free of (even criminal) consequences. That's not the case.
To start, recall that there has been an ongoing investigation into a "pay for pardon" allegation that broke in the news a few weeks ago. If Trump could give pardons with no limits on corrupt intent, with total immunity from criminal consequences, wouldn't that mean he could literally set up a stand on the White House lawn and sell pardons for a million dollars each to passersby? As weak as our laws have been in restraining Trump's urges toward complete piece-of-shittery, I think even Trump would be seriously surprised to hear that he's allowed to do that (or he would already be doing it).
Similarly, the Constitution gives Trump the power to approve and sign bills passed by Congress. Is he also allowed to do absolutely anything he wants with that power, immune from criminality? Can he non-criminally sell his signatures for cash or in any other blatantly corrupt manner?
If not, I don't see why the obvious corruption of Trump pardoning his own campaign staff, who were caught lying to impede a criminal investigation into his campaign, couldn't be the basis of obstruction of justice or other criminal charges, though it would be helpful to have evidence of more explicit communications regarding such quid-pro-quo's.
Trump has really warped our belief in the law and created a sense of powerlessness for us to be accepting his use of the presidential powers in this way.
Here's one last way of looking at it. The law empowers a person to drive through intersections when their stoplight is green. But you're not allowed to go through 100% of green lights with total criminal immunity. A person who is idling at a stoplight and punches it when the light turns green, knowing full well there is a pedestrian in the crosswalk in front of their car, can and should be charged with murder if the pedestrian is killed (or attempted murder if they aren't).
Just because the law gives someone the power to take an action it doesn't mean that power can be used without any consequence in all situations. Context and intent matter.