I think the Donks will pressure Mueller to give them something they can hang their hats on to bring impeachment charges in the House and argue that Trump should not be allowed to nominate a new Justice until Impeachment proceedings have ended as they slow walk the proceedings.
Great insight to possible demokrap tactic to prevent further SC nominations by Trump. If by “impeachment proceedings” you mean to include all the committee investigations related to possible impeachment (i.e. actions prior to drafting articles of impeachment)... then yes, it will be a very long, slower walk.