When you're fishing off Nanaimo a lot of the time you're going to want to be down at least 200 feet. Sometimes much deeper.
You need to:
1. keep your lead from down rigger clip to flasher to 15 feet or less to lessen the chances of lines crossing over and tangling (which also helps when a fish does a "pop up" type of hit down deep instead of a hit and run as there is less slack line to deal with). In my opinion 30 to 40 feet is waaaay too far back unless you are trolling only plugs on a shallow line (like we used to do out offshore in the pilchard days on the west coast) without a lot of other boats in the area.
2. ensure you are trolling with your boat "in line" with the current, i.e. so one of your rigger cables isn't cutting under the boat on an angle. Also if you are trolling with your cables on a ****-eyed angle, your two flashers are going to be rotating differently and likely 1 side will not fish as well as the other. This is a very common mistake.
3. pull your outside rod and rigger up when you make a turn, then if you want to turn sharp you can and you are guaranteed not to tangle, and I find a lot of the time popping a sharp turn on that inside rod drastically changes the action resulting in an immediate hit. If I am trolling "deep" (180 plus) or in significant current I very rarely turn with 2 rods down, even on a gradual basis. This is also a good excuse to check your lines. Nanaimo always has tons of shakers, and often lots of weeds in the water.
4. if tangles remain really bad, consider running a big plug or spoon with no flasher on at least one side, potentially both sides.
5. strongly consider getting extended booms on your down riggers. Harbour Chandler set these up for me many moons ago. Once I did that and went to 15 pound balls, my number of tangles declined dramatically on a boat with an 8'6 beam.
6. listen to experienced guys from Campbell River on this topic, nowhere else on the coast do people consistently fish in extreme tides, really deep, as the guys who focus on the south end of Quadra. Most of my experience with this issue is west coast offshore or Nanaimo area, both of which can have strong currents and a lot of deep fishing, but frankly nothing like what'd I've seen at south Quadra the modest number of times I've fished that area. If you can learn from an expert in that area you will gain a lot.
What you're going through is all part of the normal learning curve for deep fishing. Hope this helps.