I appreciate the feedback. I've pretty much determined a 3rd attribute is getting added to the game.
I ran some solo scenarios using the rule the rolled attribute die is lowered when taking the 3rd strain. It did make the game more deadly, but not in a good way. I then ran it with the original rules and it didn't feel OP.
So, right now, I'm leaning toward adding the 3rd attribute and not changing the rules other than the die sizes allocated to said attributes. d4, d6, and d8.