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From a manager point of view:

In your shoes I would make a decision to either fix/improve the situation or leave immediately.

If you decide for the former then make sure you have support from your management and own that decision. Be the one who turned the subpar division around. If it is as bad as you say you can hardly screw it up any worse. That's a privileged position to start from - difficult to fail.

I doubt your colleagues are really below average IQ - but perhaps their skills could be better. So teach them. Be the leader and mentor they likely never had. Have patience and build a team.

Fix the high impact, low effort problems first. Get some fast successes - they inspire and breed appetite for more. Don't exhaust your resources. Chip away one small problem at a time. It adds up faster than you think.

Create a vision for where you want to be in one year. Communicate that goal at every opportunity. Believe in it and other people will believe as well. It doesn't matter if you reach it within an arbitrary deadline - it matters that it exists in the first place.

Whatever you do, don't coast.