Investigations can absolutely happen without a supermajority. Tariffs are a maybe. the courts might be more likely to shut them down if they have the legislative branch breathing down their necks. Things that turn up from the investigations can be used to file new lawsuits. Dems could extract concessions during budgeting.
That said, I don't have a lot of confidence Dems will do much of anything, despite the wave. I hope I'm wrong.