Welcome to the boat mate.
The escort AI very much depends on what if any mods you are playing.
In the stock game they are pretty easy and the big mods make them more difficult.
Hard or easy escorts are more a function of experience than any thing else. The more experience you have the easier they get.
500 meters is pretty close to be having the periscope up for anything more than a quick look. They will see it most of the time if you leave it up very long. You want to clear the escorts by at least 900 or so meters.
If you're trying to get inside a convoy than go deep and silent till they get to you.
Even in heavy seas 2/3 is too fast that close, use 1 to 3 knots and make sure you have silent running turned on.
The realism and hard settings have more to do with your boat than the escorts AI. Go to custom settings and you will see what I mean.
Plot the convoy's course and set yourself up well ahead and sit and wait for them to come to you. You have a speed advantage on a convoy so use it and attack when you want from as good a position as you can get.
You should have course, speed and AoB pretty close even before they get to your position. All you will need the scope for is getting range and fine tuning AoB.
Good luck and good hunting
Magic