Hi all!
Hope I can can some guidance and also a question answered.
I picked this Austrian pine up from a nursery for cheap, but would love some quick tips on how to proceed. Also, when buying bonsai trees already in bonsai pots and not refined yet, or seemingly not very old, would it be wise to repot in bigger wooden boxes or back in nursery pots to grow?
I have found that Austrian Black Pine (P. nigra) will back bud fairly well if they are growing strongly and you pinch the candles as they elongate. One a nursery stock tree like this, I would pinch the strongest candles to about 1/3 their length, the medium to 1/2 - 2/3, and the weakest just take off the tip as it elongates.
I like to repot nursery stock like this into a wooden box with a mesh bottom. The volume of the box should be similar to the volume of the nursery pot. This starts to move the tree from a tall, thin root system to a flatter, wider system. The final bonsai pot is apt to be even smaller and thinner, but this is a good start. I like to use a 50/50 mix of aged bark and pumice for my developmental trees since it is inexpensive, allows strong growth (the bark holds and releases nutrients), and the pumice gives some structure so the root system can be worked in the next repotting. Keep in mind that they first repot from nursery stock is almost always the hardest on the tree.
I am thinking you want to reduce the height of this tree. You might as well do at least some of it now. If it is in spring growth there is lots of sap flow. Covering the cut end with something to absorb the sap is a good idea - long fiber sphagnum moss is traditional, but a couple of layers of old towel or washcloth would work as well. Just tie it on nice and snug around a stub that should be 1 - 1.5 times the diameter the cut. You can prune the stube back later.
For a new bonsai that is not yet in refinement the key question is what do you want to do with the tree? If the goal is refinement, keep it in a similar pot. If it needs grow a bit more, I would move to a larger bonsai pot (2X volume max) in bonsai soil to focus on secondary branch development. if it needs to grow a bunch, I would move to a wood box of about 2 to 3X the current pot with a developmental soil mix.
I hate to be a ‘hur dur, just watch the videos’-guy
But there is a lecture in the library called ‘defining outlier pines’ that covers the why’s of a few different techniques of Austrian pines.
Welcome to the forum Sakuku! Im with ya. I just brought home aJapanese black from the nursery about the same height and rough build as yours.
goals first! What do you want to do? And don’t say thick trunk 
that’s a trap. If you’re like me, your goal is probably first and foremost to learn à ton about bonsai. And second, you want a beautiful tree.
So the goal should be to turn this into a bonsai asap so you can “do all the things” and learn. No ground planting. Probably not up-potting either. Cuz then you’re just watering and fertilizing with the hope of one day having an even higher quality tree but at the expense of your learning opportunity cost.
I was watching a really inspiring Shohin channel on YT, bonsai shinshi. And he said, “bonsai aren’t grown they are built.” I loved that. Build your pine! Also, I’m a believer that meticulous technique and ramification drives incredible trunk girth, without sacrificing your learning along the way.
The problem is that, depending on where you live, it isn’t the time of year to do all that much on pine. Decandling is coming up, but I would leave them to pump up vigor ahead of your Fall and late winter operations.
Right now: fertilize and water.
Fall this year, trunk chop and major branch selection.
Late winter, repot. Development box or training pot could be ok. As for soil, that’s a whole discussion.
next year, profit.
That’s my plan anyway for my JBP.
