I do something similar with carboys and my conical. I have different blends of Sacc and Brett strains that I use continuously. When I brew, I bottle, and pour the fresh beer on top of the old secondary cake. Typically I throw out the primary sludge. You could do the same thing but pull out only a few gallons to bottle. Prolonged bottle conditioning is supposed to help get more complexitybut to be honest it is so darn hard to not finish it. Also, why not consider a 12-14 gallon demi-john? If you can pick it up, they can be had for like $60.
One last thought, if you want oak and gas exchange, I'll share that I was able to cut a hole through a universal stopper like this: (Universal Stopper With Hole | MoreBeer) and stuck a 2 foot long 1" diameter oak dowel through it. Filled the gap with silicone so all gas exchange goes through the oak. seems to be working nice. Been groing funky for about a year and a half. I've added fresh wort on occasion, and pulled some out to blend into other beers.