If only we had a rational 3rd party...I agree. I think the first wave is from those who can't be "hurt" by Trump. Obviously his scorn can really put the pain on a candidate, and with 90%+ approval ratings within the republican party, we can assume his opinion still has sway.
My guess though is that the second wave will be House members or other Bush-like Republicans, whom are more establishment, will come out against Trump. I think it is well-known how much a portion of the Republican party dislikes Trump, but just doesn't want the Democrats to win, and are holding their tongue. Without any clear alternative, what advantage do they have to speak out against Trump though? They would just be hurting themselves politically. The only real reason is because they just want to get a parting shot at Trump on the way out. I don't see Senators willing to do that, but House members may.
I predict that Biden will win the election because Trump is not going to change his tone or trajectory in the next few months. I also think you will have a lot of republicans stay quiet and vote for Biden just because things are so bad with Trump. I am not quite there yet (as in voting for Biden) but if things get any worse, I might be. I am really disappointed in the political establishment for not having a backbone and standing up to this tyrant. We have checks and balances in place for a reason and they are letting him get away with WAY too much.