iPhone 15 Won't Connect or Discover AirTags

Yes, you discovered one oddity with having multiple tags near each other when setting them up. Apple recommends setting them up, one-at-a-time (pretty much the only way you can anyway), but keep the other tags outside the Bluetooth range (around 30') of the phone/tag when doing so.

Did you verify that the replacement batteries you are using do NOT have a bitterant coating? These are known to be problematic and may prevent the tag from working properly.

How long have you had these tags. The typical battery life is approx. one year. I'm assuming that you purchased them new, directly from Apple. Is that correct?