Only two R seats are at risk: Collins and Gardner
In fact, AL should be a pick up for the Republicans.
That certainly is plausible now, but you started with “Even if Trump loses in 2020...” Any situation bad enough that could result in a Trump loss in 2020 could also endanger more than 2-3 R. Senate seats. But I am not expecting such a bad scenario... we just need Rs to pull together and MAGA.