Please correct me if I am mistaken, but for me, the most rigid steps to avoid have some weird, random move-ins after we have upgraded to the service centre and built the campsite would be:
1.
Do not "accept" the villagers coming to your campsite, until someone visits you like. Admittedly, this would slow down progress of the main story line (weird to write about something like this in case of an Animal Crossing game, but I guess, it is clear what I mean); there won't show up any villagers on island tours, so this can take long.
2.
After this, do not lay down the plots Tom Nooks offers to you.
Laying down plots bears the risk of random move-ins.
Visit the campground and take island tours.
When an animal suits you, invite it.
Only after this, lay down one plot, wait one day for the villager to arrive.
Repeat 2., until you have ten villagers.
Here, the problem is, that there are many different verdicts on the web, saying "you need free plots for villagers to spawn" (be it the campsite and/or the island) respectively saying "you do not need free plots for villagers to spawn".
To me, personally, villagers to also spawn without free spots would be the most sensible option, at least, as far as the campsite is concerned (else, it would be rendered "dead" as long as you maxed out villagers; from New Leaf, we know, that villagers will indeed spawn; you simply do not have the invitation-option; I imagine it to be the same in New Horizons and could even imagine it to be analogously for villagers on mystery islands, but what I imagine is pointless, we need facts).
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I personally do not care much for the "main story" line to take longer if in exchange I can make sure that no villagers I dislike move in.
Further, admittedly, also afterwards you could let villagers move out and then go "hunting" for a new villager. Though an empty spot, as mentioned, comes with the risk of a random move-in - this is something I believe in after reading it on the web.
Hence, to be patient in the beginning, at least to my understanding, seems to pay-off.
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Can someone disconfirm this method?
Either way, would be great to get more information on this issue.