I wonder what 'relatively small' means.
If the root ball is occupying most of a 170 litre pot that should be ample.
I think that these plants do have a relatively compact fibrous root system. If you've just dug one up, you'll know. So as long as there is a good core to the plant, it should live.
Bear in mind that plants grown in pots are more tender than those grown in the ground, because the roots are effectively above ground, so lose heat more easily in cold weather. I had a large trachycarpus in a pot, which I lost two Winters ago, for that reason.
Mine was only about 6 feet tall. It was in a 50cm/70litre pot, and the roots were filling the pot.
You talk about removing leaves to reduce moisture loss. I think moisture loss is only a significant factor in lush-growing plants, I wouldn't have thought it was important for a slow-growing plant.
It seems that you're trying to encourage new growth now. With a palm-type tree, I'm not sure if that's the best thing. Maybe someone else will have better ideas about that.
20 feet is very tall for a tree in a pot. I would have thought the most serious problem would be toppling over in gusty wind. Either because the root ball is not secure in the pot, or because the whole thing topples over. I used a soil-based compost, and my, relatively small, 70 litre pot, was extremely heavy. I guess that a 150 litre pot, full of soil would be unmovable, and ought to remain upright.