Hi Max – well done for sticking with the project!
I think trimming the slit will be the best approach here. It might make a slight dimple in the tube there when it’s inflated, but that part will be underwater, so it won’t be very noticeable.
If you have a seam strip that is longer than required, you could extend the slit a bit farther into the floor (an inch or two) so you can trim up to the other end of 4L and make the trimmed slit edge a bit less curved.
Alternatively, using an extra wide seam strip could help here, but honestly I think it would be pretty difficult to execute well, so trimming the slit is probably the better option.