The other options of adding more escorts few hours after the initial escort unit be under waters also migth bring some realism and increase the difficulty.
Or even the merchants engage on ramming the sub once they lose their escorts/as soon as they spot the sub
I'm sure that any addition to current convoys AI would contribute for better gameplay making the AI a bit "biffed up" and less predictable.
Otherwise a player can be within the convoy for hours and hours (like I did - 8 hours with the convoy taking on ship by ship). Because the AI would simply not react to a threat beside easy zig-zag.
unfortunatly I don't know how to program on SH3. Otherwise I would come out with something like that... For this reason that I'm offering these ideas for someone interested to pickup and explore these or other ways to enhance the current convoy reactions