I've also been playing with that one, two hacks;
That's the first one, on the left is the original course, the red circle is where it actually goes, cutting the corner on that 135 degree angle at WP5. At that location, 120.13 / 17.5 is where it stops and proceeds to circle, different ships slowing to 1 knot then randomly speeding up to 13, running halfway around the rest of the group, slowing to 1 again, then waiting until several other ships take turns playing drunken sailor before doing it again. On the right is the altered course, gotta start a new career to test that. This is Convoy12, which starts and ends near Formosa.
Second item is the number of ships and spacing;
Original
7 DD
3 CL
8 small passenger carriers
total 18 ships
My edit
5DDs
2CA
1BB
1 modern passenger liner
1 old passenger liner
total 10 ships
I thought getting rid of all those tiny 1800 ton troopships and increasing the spacing might help, looks like I might be wrong. OTOH two regular liners add up to about the same tonnage, those little 1800 ton bathtub toys aren't really worth a torpedo.