I have my boar pens in a Meadows biome far from my base and teleport to it. The key is to not have too many items that establish player base too close together to trigger events that would endanger your livestock. So, no events, no problems.

Also I have a stakewall perimeter fence around the pens 10m out from the edge of each pen so even greydwarves walking by can't get close enough to see or spook the boars. It's as peaceful as you can get.

If you're not familiar with which items establish a player base, review this:
https://valheim.fandom.com/wiki/Player_base

Just keep items on that list far enough away from each other as to not trigger events near your boar pen and you'll have a much easier time breeding your pets.