One more thing to add. The appearance can be different as well. The appearance of the first J20 prototype 2001 on the tarmac of CAC, or the appearance of the first operational J-20 in a public air show years later. Both the starting point and ending point can mean differently.
Besides that, there is a chance that H-20 prototype may be spotted some time in 2019 because around August 2018 a very large S-shaped exhaust duct for x-type aircraft was fabricated and handed over to 1st Institute in Xi'an. So the first prototype of H-20 could be in the process of assembly right now. But I don't think XAC is in a hurry to have it taxing on any PLAF or PLA or National day unlike induction of operational piece.
It being a nuclear capable bomber means that it’s development path may be more similar to DF-41 or Type 094. That is, we don’t see much of it until it is on the verge of entering service.
I believe that prototype(s) do exist, PLA has some experience with flying wing through the sharp sword drones, and like you said in your post they premade some parts for the plane as far back as last year. Since it’s not an aircraft carrier, it should not take an entire year from part delivery to finish building.
But these prototypes will be kept out of public sight unlike the J-20. They’ll only be shown once they are at least finalized.
That said, is the general consensus that this will be a pure bomber with flying wing design and not a “JH-20” type that is to the J-20 what the JH-7 is to J-7? China is apparently making a variable cycle engine as well which would prove useful on a “5.5gen” striker