But what would you say is the most consequential use of DoJ - whether successful or thwarted - he tried to do to go after someone else in his first term?

How about the unusually prolonged and fastidious IRS investigations of several of Trump's then perceived enemies? Can't recall all their names, but I think James Comey was one of them.

PS - I agree with you that this was a very disappointing thing for Biden to have done, and frankly, now it stinks to high heaven that the only purpose for Biden having declared - multiple times - that he would NOT pardon Hunter was to attempt to win votes for Dems by painting them - and himself - as the party that obeyed the principal of non-interference with the work of the DOJ, no matter the outcome. And his explanation for why he changed his mind stinks of grasping at a convenient excuse: the election of Trump. I suspect Biden intended to pardon Hunter all along, even if Kamala had won.

Sadly, Biden's legacy is now further tainted, and as you said, this pardon only ensures Trump will proceed with renewed vigor with his plans to weaponize the DOJ - or what's left of it - against all "enemies from within" for his own nefarious purposes.