Arnaud: If you want to make it a substantial tree it needs to go in the ground, and grow for a few years. These grow like weeds capable of putting 2-3 feet a year easily (here in zone 7). And what Shibui said.
StickyGecko: you probably have a couple of layers of roots in the pot. Nursery DawnRedwoods - especially if they’ve been growing in the same pot for a while will rapidly fill out the bottom of the pot with roots, and then readily ground layer and form a new layer of roots close to the surface where they have access to oxygen. So my suggestion is to repot this next spring to first figure out what the roots look like. You can bare root it and aggressively cut back roots when you repot. You probably have roots going straight down, and you may need to put it in a flat tub that is kept soaking wet. You can drill holes at the edges so that the roots can escape into the ground. It will be a lot easier to transition it back into a smaller pot. Look up bald cypress techniques and they should work well.
You’d be fine to airlayer next year as soon as the leaves flush out in early spring. They airlayer very readily. If you do that tho, you’ll be setting back the increase in base of the trunk - so you may want to think about whether next year is the appropriate time for you to airlayer, or if you want to wait. Airlayering in two or three years after moving it into a flat tub (or the ground) will give you a not only faster root flare, but also an already fattened airlayered top. For the rest of this season, feed well.
(Also, maybe start a progression thread for this tree.)