The Republicans will be able to obstruct by a cunt hair, but obstruct they would be able to. Biden won't be able to pass any legislation or spending bills of any policy agenda consequence without control of both chambers. He will still be able to fill vacancies and appoint judges at all levels of the federal judiciary (including SCOTUS should spots open via croak) and the Republicans can do absolutely nothing to stop it.
It's actually pretty damning that the GOP couldn't win the Senate. They are statewide offices decided by popular vote so there's no way to gerrymander them ala House of Representatives districts. However, as you're likely already aware every state in the union gets equal representation with exactly two Senators regardless of population size. So deep red states such as Wyoming (581k), North Dakota (770k) and South Dakota (896k) are on completely equal Senate footing with blue states like California (39.6 million), New York (19.2 million) and Pennsylvania (12.8 million).