Between a friend and I, we have 4 "big" barrels.
Two of them are group barrels. Every year or so about 8 of us get together, everyone brings their share of fermented beer (5-10 gallons), we bottle the beer currently in the barrel, rinse, and refill.
The other two are soleras. Every year to 18 months we brew ~25 gallons of beer (multiple boils), transfer out 20 gallons, and then refill with the freshly brewed wort. We’ve been bottling 5 gallons plain, and then doing fruit, dry hops, and herb/spice on the rest. The extra 5 gallons is to account for evaporation and to have a bit extra to top-off after six months or so.
Those seem like the two most manageable ways to deal with a big barrel to me.