Barrel headspace secondary concerns

Greetings. I've been digging around on this topic and haven't quite found what I'm looking for. I might not be asking the right question though .

We have a 30 gallon used wine barrel that we filled by doing three 10 gallon batches in one day (that was a good experience in itself).

When we transferred from primary to the barrel, we ended up a few gallons short due to trub loss, etc. The barrel will likely soak-up a bit as well. What we're left with is a decent amount of head space at the moment. Since we added bugs to the barrel, we're currently banking on an additional layer of CO2 to occupy the space for now.

Longer term, any thoughts on if we should be topping off? Do the Bret elements want the oxygen?

Thanks in advance! Additional info about the factors/recipe if useful here.

In case the thread/topic proves relevant to someone's search in the future, we elected to give the headspace a little time in the barrel under the assumption that the CO2 will occupy the space, then dissipate through the oak and airlock.

We've since brewed a 10 gallon batch of the same beer. Half of that batch is currently fermenting using the WL Belgian Sour Mix as the primary yeast. That one is definitely a first for us, but we have time and it will be added to the barrel that already has about a month of a head start on bugs freshly added to the secondary at the time of transfer. Somewhat surprising, we have visible attenuation at about 24 hours.

If anything else relevant happens during the course of this experiment, I'll update the post for the (hopeful) edification of others' adventures.

If you are fermenting in a barrel, which it sounds like you are, you don't have to worry about headspace too much now, as the CO2 will fill it. But once fermentation is over you have to either flush it with Co2 daily or get it out of the barrel.

If you are aging it in there after fermentation, you want to be topping off at least weekly. Generally you would have 5 gallons above what you need to fill the barrel to do this.

Thanks Jason. Primary fermentation was done in 6 vessels (4 glass carboys and 2 plastic primary buckets). The barrel is our long term secondary vessel.

Just topped it off with 5 gallons of the same recipe that we used WL Belgian Sour Mix as the primary yeast. Hadn't tried that before and if anyone's wondering, the apparent attenuation was good reasonable and we dropped almost all the way down in about 7 days. Normally we use this in the secondary (and did in the barrel). Thought that was interesting.

When we topped off, a pellicule was starting to form. Not sure what happens with that now that much of it is coming into contact directly with the oak now that the barrel is full.

If anyone sees any red-flags in the steps to-date, by all means we'd love to hear it. Fingers crossed to the extent that it might be beyond our control at this point!

On first sampling after topping off, so far so good. Now we wait until the beer tells us its ready