I personally started designing my town when I had about half of my dreamies, and it was quite the mistake. It made plot resetting that much harder because I already had finished areas of my town that they would move on top of. For my next town, I'm definitely going to wait until I've gotten all 10.
Definitely after so you don't have to erase any work if something doesn't work out. It will be worth the wait and save you some grief if something were to go wrong. Think of it this way: after you complete your dreamies then you can really work on creating the most perfect dream town you envisioned for your perfect dreamies to live in. I did it this way and i wouldn't change a thing.
Definitely after. I made sure to get at least 8 of my villagers before officially designing and thought that that was okay, but then the plot resetting for pekoe and Cousteau was a nightmare. Random unwanted villagers before them moved in and out, lost dreamies because I stupidly didn't plot reset them right, destroying and rebuilding pwp multiple times to make everything look right, rearranging paths, fixing horrible dirt left from all that, just a slew of messiness. If you do start designing your town like I did in the middle of getting everyone, make sure to design was lots of space in mind. Otherwise, you're gonna have to plot reset a lot to get them in perfect 3x3 spots.
I would recommend after. It's also easier when you've unlocked most PWP's and catalogue items.
That way you don't have dreamies moving in on wrong places when you plot reset and you can create a better image of your dream town having all the PWP's and items in your head.