Any advice for what to do with it from this point? I was thinking chop at that low branch and put it in the ground.
My first thought is to wonder how bendable that trunk is, down low - probably very difficult to do at this point.
I agree that the beginnings of your tree are probably at that whirl where you now have two branches (indicated by arrow 2). I see there is a new shoot near the ground level, which may be the lowest node on your tree - how far down is it from there (arrow 1) to the roots? I think this is a node because it is extremely unlikely to pop a bud in the internode (the stem between successive whirs/nodes such as between arrows 1 and 2) after the needles have fallen off.
So, one of the two branches at node 2 will remain a branch and the other will, at some time in the future, become the next trunk section. Either way, you want to be sure to have a node close to the trunk. Nodes/whirls are created by the seasonal terminal buds. Usually the next season's candle has a 'neck' and then a stem covered with needles. That branch going to the right of the trunk looks to have two nodes, but I cannot say for sure. As annotated on the pic, I would prune this branch at the arrow next spring, when the buds/candles have started to elongate. Often times the tree will generate terminal buds at the cut end (i.e., create a node), sometimes it doesn't. It likely will release a fascicular bud or maybe a few - buds at the base of a needle group.
The trunk above node 2 will likely be removed, but you will keep it until the trunk (1-2) is the thickness you want for your bonsai. Planting it out will help with thickening the base (a natural reaction to the long sacrifice waving in the wind with the roots locked into the ground), but it comes with potential difficulties getting the roots out of the ground afterward. It would be good to spend at least a couple of seasons developing fibrous roots close to the trunk before sticking it in the ground; i.e., grow in a pond basket full of inorganic medium first. Once the trunk (up to node 2) is as thick as you wantn you'll chop the trunk and you'll point the end of one or the other of those 2 branches up to make your next trunk section. If you planted it out, this would also be the time to dig it out again and return to growing in a pond basket.
Lastly, there is a lot of foliage above. Removing the top whirl wouldn't slow down your trunk thickening very much, so you could try layering it while developing roots. In my cool climate, it took most of three seasons to air layer a JBP 'thunderhead'.
George Muranaka in Nipomo, CA gets harvestable layers in one season; you likely will too., But, so what if it does take two seasons - you're developing roots during this time anyway. On the other hand, it is no biggie if you just don't want to mess with it.
Do investigate how far down it is to the roots from that lowest node 1. It may be appropriate to shift down to it being the node of your next trunk section instead of node 2 as I've discussed.
... or something along these lines.