I can't put any Gloves items in the offhand glove slot, is this intended for balance purposes and I should just accept it as it is, or is there any way to config them to be placeable there?

You are in fact correct that this is an intended balancing act. We however won't stop you from allowing it yourself, as Trinkets is configured through datapacks - so you can easily change it. You can read up a little on how the data format works in their wiki, but in the end you simply need to add the items you want to the trinkets:offhand/glove tag. In case you're not familiar, the process of creating a datapack as well as tags is outlined in this article.
Cheers

Thank you for the information and instructions :D