Why would there be a lawsuit? Both companies use a 3rd party licence of the same game system. Larian just got more of the lore and published subclasses etc.
There is literally no ground for any of them both to sue over 5e rules.
You literally wrote "the ideas of Solasta", not referring to the 5e rules, but things that were inherent to Solasta, making them not part of the D&D license, but the makers a of Solasta's intellectual property.
