I'm guessing Rick Steves would say a 21 day trip to Italy is not ideal, but for a trip that length, his itinerary is his best. That's why it's listed as "Italy's Best Three-Week Trip" in his guidebook. If it wasn't, I am sure he would recommend otherwise. I have his book right here and I don't recall anywhere that he recommends slowing down or that his itinerary is too much for independent travelers (and it isn't - I've basically done it). Instead, he constantly suggests getting the best bang for your buck and making the most of limited time.
And let's be honest. It's really not that hard. Even for big cities like Rome, you can get off a train, get to a hotel in a suggested Rick Steves home base and off sightseeing in under an hour most of the time. Worried about finding a place to eat? Make a reservation beforehand at a recommended Rick Steves restaurant. Or be adventurous and try the little hole in the wall on a random side street. Making early reservations at big sites? Rick recommends doing just that before arrival for smart touring. It's all about preparation (and a little flexibility when things don't go exactly as planned).
Sure, this is worth the conversation, but I'll bet most people suggesting to slow down have more time as retirees or have had the good fortune of being able to visit Italy multiple times. That's a nice luxury to have. But I'm guessing that's not the OP, who is trying to make the most of his 21 days and doesn't know when or if he'll be back.
Frankly, suggesting the OP is better off watching videos instead of attempting Rick's best 21 day trip is both laughable and wrong. There can be a big difference between rushing and making the most of your time. Yes, we all have different travel styles, but Rick lists his "best" itinerary for a reason.
As for the OP's itinerary, while not necessarily answering his train question, if flying in and out of Milan, you might consider taking the night train from Venice to either Rome or Naples/Sorrento and then working your way back up the Western peninsula. That could eliminate some back-tracking and save a day for better touring.