Either your disguises are effective, or if they fail, fight it out. And having a hull that looks like a cruiser's but is a freighter on top, and any warship skipper's going to be suspicious. Let alone a merchantman's. They'll think that any ship with that kind of hull is automatically a raider in disguise and raise the alarm.
It is not an either/or situation. I gave a design for a light cruiser that can operate from fortified ports in Africa, and directly challenge the British Navy. Even without colonies, these type of ships would have been useful to the Nazi's. If the Nazi had 10 of these types of ships around the world when the war started, and 10 in reserve in Germany, they could have caused huge initial interruption to the UK trade. Now with 6-12 months, the RN probably has hunted down the original 10 ships. After the fall of France, they could have been station out of France, forcing the UK to use a lot of assets dealing with them. They would compliment naval aviation, U-boats, E-boats, minefields, and larger surface fleets. They are designed to be cheap enough to be willing to lose them.
If one wants an AMC, not a CL, the specs do change to what you are suggesting. No two ships look a like. I want the RN to literally need to stop every ship it sees to search. This in and of itself is a huge gain in slowing trade. I will just make German ship yards build ships that look similar to common designs and are easy to arm. They should also be easy to modify to look like multiple classes. Today a standard USA freighter, tomorrow a Argentina refrigeration ship. Now for the new target specs.
Tons: 3K-10K. It is nice for space for extra fuel, marines and the like.
Speed : more than 10.
Guns: 2-6 15cm guns, but some may have only 10.5 cm guns. I like torpedo tubes also.
Seaplanes 1-2 if possible.
Range: As much as possible.
With a wise construction plan, many of these ships are not custom ships, but merely freighters that are quickly converted. For example, as Hitler looks for Poland in 1939, the Navy begins to trickle out 2-4 AMC per week to distant ports. The guns are either in the cargo hold and can be quickly installed OR they are hidden. By the time the war starts, there are 20-40 ships that immediately start raiding. If the war does not start, they simply completed the cover merchant mission and return home. For example, deliver the banana to the USA, and in the hold will be 3000 tons of fruit, not the 5000 tons that could be carried if their was no ammo or disassembled planes.