It’s odd, because what he described in the answer to the question in the linked article was exactly how multiclassing spellcasters works in 5e
It feels like there’s some miscommunication going on - my gut feeling is that spellcaster multiclassing is going to be very similar to 5e base.
I went all in with Glut because his story would have resonated with my PC, but all you have to do is finish off the other Sovereign and the fight ends - Glut takes control and only one myconid goes down, the rest all start hailing their new king.