A 1 - 2% per month reduction in Battery Health is normal and expected. Your phone is showing what would be normal health for a 1 year old phone.
As mentioned above, Apple would ONLY consider replacing the battery under warranty IF Battery health had dipped to 80% or less within the first year. This is not your situation and since your current Battery Health after a year would be considered normal and within acceptable range, there is nothing wrong with you battery.
There is no way to stop Battery Health from declining. The fact is Battery Health declines with every full cycle charge. This is what batteries do. ALL BATTERIES.