It's suspicious that Biden had hundreds of thousands of solo ballots, but not proof of fraud.

Here is how to prove fraud:

Check the distribution of when those ballots were counted. If it skews late, it's 100% proof of fraud.

If many people legitimately just voted Biden there is absolute certainty these ballots would be counted randomly and evenly distributed throughout the count.

There is no reason a mail in voter would be more likely to ignore down ticket than an in person voter, assuming both are voting Biden.

However, if they had to fill these out late because they were behind, they would have all been counted late, and we'd clearly see a heavily skewed graph of these ballots coming in at higher ratios after Nov 3