This is a very endgame problem and I don’t think there’s a good solution that doesn’t also create a “rich get richer” scenario or make it harder for “young” players to catch up.
Gold definitely has sinks in the forms of guild tasks. It’s the first currency you “stop caring” about. Once you level all your kingdoms, the only real use a player has for gold is guild tasks.
Souls take a LOT longer to reach that point. Just getting all kingdoms to 5 stars takes a few months. Farming for Dawnbringer and Xathenos takes a while longer. Then you need ~1.3m and increasing more to finish leveling all of the troops. After that, it’s easy to profit souls because you only need to level the 2-3 troops that release each week, and new kingdom releases are rare enough you hoard “enough”.
All I can think of is something like guild tasks for souls, but it’s a toughie. It takes a “short” amount of time for a new player to spend the gold they “need”. It takes a much longer amount of time to reach the soul threshold. So if there were soul-based guild tasks, there’d be increased pressure on mid-game players to abandon goals like Dawnbringer in favor of helping their guild. It’s already tough to convince newbies to spend their gold on kingdoms, not LTs.
Right now I’m OK with it. Getting to “I don’t need to farm souls anymore” is a very endgamey moment, and it feels good to be there.