Yes, the extent of Russia's interference -- and thus Donald Trump's illegitimacy -- is still a big unknown. When the Republicans were in charge of the White House and both houses of Congress (2016-2018), they quite forthrightly didn't want to know.
We KNOW, from the Mueller report, that some sizeable percentage of voters were swayed away from Clinton by Russian propaganda, either to swing their vote to Trump or not vote at all.
We KNOW,
from the Mueller report, that Russian hackers broke into several state voter databases and the DNC servers and state elections boards.
What we do not know, is if they hacked into the
PES vote tallying equipment and altered the vote totals. We don't know if they removed Democrats from the rolls of registered voters. We don't know if certain counties went uncounted or other counties were counted twice.
We also have no idea how many voters either didn't vote, or changed their vote, due to Russian interference -- nor what this would mean to the legitimacy of the election.