You don’t need to make a reservation for the park you hop to, only the first park. And you are correct in that “first park” choices may be limited (or non-existent) at the last minute or during peak periods like holiday weeks. But there were no reported problems with park hopping during the super-busy Christmas/New Year’s week that just passed.
When park hopping was reinstated during the pandemic, Disney created a Capacity Line (407-560-5000) for guests to check availability for the park they wished to hop to. This was due to low attendance restrictions which have since been eased. And while the parks are still not at “full capacity” due to staffing shortages, the chances of being denied entry at Park 2 are very slim.
To my knowledge there have been no reports of anyone (on or off property) being prevented from getting into the park they hopped to, with exception of Oct. 1.
I recall reading that if a guest with an ADR at a different park was unable to enter due to capacity, they would not incur the $10/per person no show fee. And anyone with a prepaid reservation would be refunded.