I'm often join servers on day 100, day 1000, and even day 3000, and as I noticed there's no issues with zombies at all. Spawned sleepers is non ferals, same as walking on streets. And first bloodmoon hordes are based on your level and gamestage too - no irradiated zombies, low count.
P.S. Noticed one issue with profile reset. When Im doing no death challenge, I asked admin to reset my profile once, so I think it reset only level, but not gamestage. Because after resetting I was lvl1 but there were many sleepers ferals , like before reset.