16x100lb is an excellent loadout, just go for the convoys and spam them with bombs(when done properly, you can easily take out 5-6 targets in one run), then quick turnaround, wait a minute, come back and bomb the second convoy, if its still alive that is. Then its just "precision bombing" leftovers (with good aim, AAA, artillery and light/medium tanks go in one bomb) until you get shot down. I wouldn't bother with 4x500lb bombs, too small to take out pillboxes, too big for light targets, and its better to just get torpedoes for the ships (1 torp. = 1 destroyer)
Thats at least how i do it: one Catalina(PBA5a, the heavier one) with 16 bombs to kill as many light targets/convoys as possible, and then finish off heavy pillboxes(they tend to stay intact till the end of the battle, since nobody can kill them) with 1000lb bombs on the other Catalina(assuming I'll manage to survive ofc.
).
Killing many light targets is much better than trying to kill the heavier one's when you don't have any big bombs. Because this way you can get massive XP from bonuses (+500 for killing 2 targets in a row, then another +500 for killing 5 ground targets etc.)
Germans can really use that to their advantage, since they get 32x50 kg bombs on their HE 111's, double the load of a Catalina. That must be fun