it’s not much of an issue. If you are ageing for any length of time in plastic you will get o2 pickup through the plastic anyway.
That would be the reason to secondary in a corny…if it were air tight.
or not to secondary. but yeah keg is a good place for ageing. On the other hand most of the styles that actually benefit from ageing also benefit from some small amount of o2 ingress so I like glass with a silicon breathable bung. it minimizes o2 pickup while still allowing for some micro aeration during ageing.
What I’ve been doing lately is fermenting 6 gallon batches in 2 cornies, then transferring and combining them in the serving keg. It’s been great. Just bend your liquid dip tubes on your fermenter cornies so there’s about 1/2 gallon deadspace and you don’t have to worry about trub pickup. Yeah, it’s two vessels to clean, but they’re really easy to clean and make sure they’re sanitized.
This way, if there’s any beer left over after transferring to the serving keg, I can use a couple grams of honey in a bottle and fill with a picnic tap/bottle wand and bottle condition what is left over. That’s been really awesome too.