I have been looking for a way to change the loot tables of the treasure chests but it doesn't seem to be possible with datapacks. I would really like it to be so and I think it would be a great addition to how customizable it is, given that you already can configure it's difficulty

There is a loot table that you can override or modify you can read about that here: https://brownbear85.github.io/stardewfishing-wiki/technical/datapacks/