Ohio, zone 6, Tamarind are a good indoors for winter, outdoors for summer bonsai. Are you able to put this outside, on the patio, or a balcony for the summer? 100% indoor growing will be a long slow process. Quicker if it can get outside part of the year.
Thickening the trunk is a function of the total surface area of all the leaves. The more surface area of leaves a trunk supports, the quicker and larger the diameter of the trunk will develop. 2 styles of doing this, let a sacrifice branche get tall, often 5 to 10 times taller than the final ideal size, or to allow many branches to develop, and "hedge prune" to keep it small enough to go through doorways and fit in where it needs to be wintered. Search threads here on BNut and read Walter Pall's blog about hedge pruning. It is a valid technique. Keep stepping the tree up into larger and larger nursery pots until the desired trunk diameter is obtained. Then radical pruning is done, and the tree will be brought down in size to the desired size for bonsai. Most bonsai are brought down to size. It is very rare that they are grown up to size. I have an Amur maple that will become a shohin, less than 8 inches tall. In year 3 through 6 it was over 5 feet tall. At one point it was over 6 feet tall. That was what was needed to get a trunk greater than one inch in diameter. Now the sacrifice branch has been removed, and it is becoming a small tree.
Since you have to bring it indoors for winter, you will not be able to let it grow very tall. So you should encourage it to make many branches. I recommend "hedge pruning" when it becomes too large to go through doorways easily. To develop a thick trunk you have to let the tree get much larger than your ideal "finished" size. Encourage many branches, you want leaves everywhere. This will thicken your trunk. Once you have the trunk the diameter you need, you reduce the number of branches and shorten the branches.