I'm not sure I quite understand how you'd get a trader to visit other than trying to deepstrike and intentionally create pockets of AI worlds that the trader has no path to get to other than through a world controlled by you, otherwise they'll route around your empire.
In general, it's expected that you're not taking every planet, but are hopping around with gaps of 2-3 planets of the AI between your planets. Otherwise you're really wasting AIP. It's not even something I consider deepstriking, that's just good 'ol planet hopping. If you do that, by the mid-game you'll likely have a
lot of paths between AI planets that cross your planets. Just not your core network.
Also, does the trader choose the AI world it paths to at random? If so, say you create a pocket of 3 AI worlds, you'd have a 3/(80 - number_of_player_worlds) say the player has 10 worlds here... so 3/70 or a 4% chance the trader could come visiting each time it changes it's mind as to destination... seems like you'd be lucky some games to ever see the trader, and thats only with extreme effort on your part. I think I'm not understanding the mechanic fully at all, this seems like a nerf with a 20,000 mile wide bat which isn't usually you guys' style.
Yes, it's random. But you're assuming that you've left one AI planet inside your network. In that case, sure, 4% sounds about right. But with anything that's random, that could actually mean zero visits or a ton of visits, right -- it's not evenly distributed, it's random.
But, as per my notes above, it's assumed that you're hopping around planets at least some, and even if you're not you're going to wind up with inevitable open flanks on the sides of your outermost planets where there are AI planets on both sides. In those cases, the likelihood of trader visits can skyrocket. In a lot of maps, that can actually lead to closer to a 50% chance per trader re-pathing (which, incidentally, it was always a flat 50%).
If you just turtle in a corner, the trader won't notice you, this is true. But assuming that you take any planets at all ourside of your safe little corner, all sorts of possible AI paths will suddenly be crisscrossing your planets. The net effect and goal here is to make the trader not visit the turtle zones, and also to not visit you much until later in the game. Along with the other stuff already noted in past posts here.
Hope that makes sense.