I did part of it from Astoria to San Francisco in end of May and most of June in 2014. Rarely, we saw people riding from south to north, they had our sympathy.
Starting in May, bring good rain gear.
I wrote up my observations that now are over a decade old, so many are no longer pertinent:
Southt to North Pacific Coast Highway Concerns
You should be able to take Amtrak back North when you need to. I have not riden Amtrak on that route, so I do not know which method they use for bikes. Different routes use different methods. Figure out which route you might want to take and which bike method that route uses first. Then use this link for details.
https://www.amtrak.com/bring-your-bicycle-onboard
Some routes require that you make a reservation for the bike, it might be harder to find a train with room for your bike than it is to find a train with an open seat for you. If you have not ridden Amtrak before, a key point, some Amtrak stations do not handle luggage. You want a station that handles luggage.
Amtrak allows two checked pieces of luggage and your bike would be one piece. So, the rest of your checked luggage has to be one item under 50 pounds. Photo below is from the Amtrak station in Chicago, the yellow duffle on the bench and the handlebar bag on the bike are my two carryon items, the red mesh duffle on the floor holds my four Ortlieb panniers. Thus, my four panniers were one checked item, and my bike the other checked item.
In this example, I was on a route with train side checked bike, I rolled the bike to the luggage car and lifted my bike up to the Amtrak employee in the car.
Regarding choice of bikes, your rear panniers have to be far enough back to give you room for your heels as you pedal. Thus, a good touring bike has longer chainstays than most road or gravel bikes. I have three touring bikes, the photo above is my light touring bike and it has the shortest chainstays of any of my three at 445mm. But a lot of bikes you look at would have even shorter chainstays. Having your weight further back than that can impair handling, as the tail can wag the dog if you have much weight in the rear panniers.
And a good touring bike has a really wide range of gearing.