From what I understand, the slowdown has nothing to do with the artists. In fact the art is one of the first things that gets finished, and that's true for a lot of projects. The reasons the updates have slowed down completely falls on Zion and finishing the actual game. To put in perspective:
Side Dungeons and Summer Areas were finished on - 08/09/23
Winter Area was finished on - 08/22/23
Autumn Area was finished on - 09-08-23
and that's it. Since then he's announced their other side project they're working on. I truly don't think he understands how all of this looks. It may not compute with his mind that Chapter 5 has taken the longest to come out by a considerable margin, and his fans and supporters have been waiting a long time. He should not be talking about working on side projects, not at this stage. He should be hyper focused on getting Chapter 5 done.
He's a man with big ideas, poor project management skills, and an inability to read the room. I'm saying this as someone who doesn't dislike the guy, and I love Succubus Covenant.
There are many reasons for the massive slowdowns we've seen in the production of SC. Lack of creativity, side projects, overburned, lack of artists and many more. But like it or not the most logical reason you can find is laziness. I've said that before and my comment got deleted for some reason, but dude has a steady income and his incompetence doesn't affect in any way his income. His patreon fans seem loyal enough, they do not care about the production time as long as they receive their promised game. Zion is a business mastermind. Works whenever he likes and feeds to the fans the dumbest excuses to ever exist when he doesn't meet his deadline. The demand is big and he abuses that to the limit. 2 years for a single chapter. It's nothing but laziness. The only single reason is laziness, literally nothing else.