Exits aren't great in the best circumstances, but early unweighted exits are widely believed to be almost completely useless.

I suggest looking at states like Oregon, Washington, Colorado, Utah, etc. that only do mail in voting and see how they measure what used to be measured by exit polls.

> Are there some other measurements based on something else

For 2016 and 2018, Pew had validated voter datasets, based on people in their American Trends Panel who were matched against voter databases and thus known to have voted. Assuming they do the same thing (or some other reputable firm does something similar, or ideally both) for 2020, that would probably be the best data source.