That current pot is plenty big enough.
The problem is NOT too small of a pot.
The problem IS too much upper crown foliage.
That big upper crown is shading out all the lower branches. Without sunlight the lower branches will wither and die. This is true too for any interior branches on the upper portion of the tree. Foliage close to the trunk, but under the circumference of the outer canopy, will die off. This leaves you with a ball shaped exterior with a hollow interior, with long spindly branches looking like the spokes of a bicycle supporting the green canopy.
If you want to turn this into a bonsai, it needs selective pruning to open it up, key branches selected, pads of foliage created, etc. This will allow sunlight to enter the interior of the tree, stimulating backbudding.
From your questions and discussion on this thread I can tell you do not have the knowledge and experience necessary to do the things I just described on your own. I’m not putting you down, I’m just saying you don’t know what you don’t know.
Either taking this tree to a workshop or having someone come to your place for a styling session would be the best thing. This tree is far overgrown and it will take some time to bring it back. For this, experience is required to know how trees will react to heavy pruning. When this tree gets worked properly, it will not immediately “look pretty”. But the structure should be placed so that as it fills back out, it will start looking better.
I remember when I gave a class on JBP and I taught the students how to decandle. And pull old needles for balance. Quite frankly, many of the trees looked like plucked chickens when they were done. To one student I said that their tree looked really good! To which they said, “Really? I spent $200 on this! It now looks like trash!” I told them, “Just wait. Put it in full sun, start fertility in a month, keep it watered. Bring it back here to my next class in November, and if you don’t like it, I will buy it from you for $300!” Well, they did. Bring it back for the next class, that is. But they would not sell me the tree! Their words, “I would never have thought it would do this...”
My point is, seek expert instruction.