I used the diving trick to get all of mine, I literally unlocked them all within about 3 months. I used to isolate the personality types I wanted the pwps from, so I would dig holes around them near to the beach and then go diving. As long as they're near to the beach so you can avoid other villager pings, at some point you should be getting pwps requests from them. Most of the time I was managing 1 a day. It can be time consuming, took me a couple of hours of the diving trick sometimes, but it always worked
No problem you can only get a maximum of 1 suggestion a day, so me being as impatient as I am decided to time travel to get several in a real time day. So you always have that option too best of luck!
It is totally random. Once in a while they will ping you, and there's a chance one of the things that trigger the conversation is asking for a new PWP.
But keep in mind only specific type of villager can suggest an also specific PWP. That's why is best to have one of each (there are eight types of neighbours).