Looking good. Great for a first attempt. I would just leave the arrangement for at least 2 years just as it is.
Tips
The 2 little ones in back, right spot for them, they should slant slightly toward the back even more, to extend the sense of depth.
The little one right next to or sprouting from the front left birch, is great, it slants to the left, out toward the edge of the pot, gives some sense of movement - good.
You have the thinnest trunks in back. This is good.
So you have done most of the ''checklist'' correct. Visually it looks good.
I know you probably have not adjusted heights much, these are very young whips, and need a few years of growing.
This year mostly just grow it out. About a week or two before the summer solstice (a good marker for those outside the USA) or sometime around Father's Day (for in the USA), I would do my first pruning to shape the trees, get rid of totally out of place branches, If any branches have overly long internodes, cut them back to the last short internode. Think of the placement of branches as if this were one big tree, at least in terms of fitting branches together. If branches jam up against each other, keep one, eliminate the other.
Allow the thickest trunk tree to have more branches and leaves than any of the others. Keep the thinnest trunks sparse, with few leaves. This will speed the growth of the thickest one, and slow the thickening of the thinnest one. In time, it will exaggerate the differences in trunk diameters.
Consider to continue to pick up more seedlings of the same type of birch, maybe every other year. For example I have seedlings of Dawn redwood mixed into a forest that are about 8 years, 5 years, 4 years and 2 years. This way trunk diameters are more varied. When you repot, 2 or more years down the road, you can pick and choose between the seedlings of different ages to switch in and or out of your forest.
So all in all, a very good start. Please post an update around the end of June, and at the end of the year.
