dnd 5th – Is it possible to build a custom weapon, and if so, how can my character use it?

This is not very embarrassing.

About mechanical balance: The short arc effectively inflicts the same damage as the handbow, which you can use with a shield. The only drawback is that you can not reload a one-handed crossbow. You can also do this combination with a hand ax. In this case, the warning is that the short arc has an effective range of just over twice the distance.

So, when balancing, the question is: will an extra distance of 50 feet break the game? OR is it a possibility to reload without a free hand to break the match?

I would say no, and I know players who are already fighting with Crossbows and still manage to charge them without handsfree.

The short bow is already a simple object, so even if it has been raised to a level at Martial, you can still use it. I could not imagine it becoming more complex like that.

I would say that it would be a more negative balance than a positive one, because I assume that you are taking all the features of Shield and Bow, which means you can not attack melee with a bow, and you need to do it. a complete action to remove a shield. . This means that if someone charges you with melee or attacks you, your character will be in trouble. The bow-shield would be so heavy that you would need two hands to handle it, and I doubt that you can grab a sword with your left hand, considering your imbalance with two weapons in your main hand. .