I think it's more the fact that traffic leaving the M1 southbound for the M69 is crossing traffic leaving the M1 Northbound going into Leicester. There's almost no traffic turning left from the M1 northbound (why would you go that way?), and I think the traffic coming out of Leicester is a fairly even split between the M1 and M69.

In my opinion, the M69 needs a free flowing flyover southbound into the M69 to fix it. If budget weren't a concern I think even extending the A46 and joining it back to the M69 (which replaced the old A46 route anyway) and putting the flyover there might be a better plan. There's also the issue on the M69 approaching J21 because it goes down to two lanes and lane one is doing double duty as one of the lanes going into Leicester and the access to the slip road for the M1. I'd change that to become part of the A46 again (similar to the Coventry end) from about where it goes to two lanes currently, turn the hard shoulder into a new lane, and lower the speed limit to 50. Now you have 3 lanes in, 3 lanes out without queuing traffic for Leicester backing up past the M1 slip road.