Yeah, cash payouts are extremely stupid. At what point does someone qualify as an "African descendant of slavery?" When they're 1/4th black? 1/8th? Or do the Democrats plan to start going by the one-drop rule again? Then of course there's the question of how to give reparations to African immigrants who may not be descendants of slavery, yet who might've been affected by Jim Crow, etc. It's an obscenely bad idea. I think the plan to put that money into predominately black schools is much more feasible, and it wouldn't result in creepy racial testing to see if anyone qualifies.
TBF, candidates aren't endorsing reparations
per se, but instead are endorsing creating a congressional committee to answer the questions you are asking.
I find it unlikely they would reach any sort of meaningful or satisfying answer to any of these questions, but I think the real purpose of the committee is to use congressional authorities to investigate high-profile research into economic consequences of slavery and its aftermath. It's a rather ambitious (and politically toxic) project but nobody is advocating that we enact reparations in 2021.