But now, with the wines two or more months into aging, I
open them up about once every 2 to 4 weeks - again, to check by nose as
well as to top up.

I've heard recently from a commercial winemaker that he tops up every
2-3 weeks. He is concerned that if he opens it too often, he is
exposing it to more O2.

That sounds about right to me too. I would make sure I used a really
tight bung too; I got rid of the wooden one. A well sealed bung will
actually create a partial vacuum. I wish i could get my hands on a
small silicone one but i use a rubber one now. I have a tiny barrel,
5 gallons. I just don't rotate mine.

Joe