Are you aware that five states have conducted their elections entirely by mail for many years, all without incidences of mass voter fraud ever occuring? Or that in the 2020 election, there were 239,247,182 eligible voters within the US and of them, only 159,690,457 actually cast a vote (65.6M of which - not 80M as you assert - were done by mail in ballots, representing approximately 41% of total ballots cast. Of those, over 22M votes originated from states that regularly vote by mail in every election, so the true percentage of those that newly adopted the practice in the 2020 election amounts to approximately 27% of the entire votes cast.
Yes, there have been proven cases of election fraud in the last election (as there are in every election but those numbers are always minimal) and to date, the only provable cases that I'm aware of that have actually led to criminal indictments have been 5 in Pennsylvania, another 5 in Georgia, a guy in Arizona and a handful of others that are still under investigation. Notably, eveyone of them favored Trump.