Hey Mitch: I paddle 10 months of the the year in Michigan, incl. the Great Lakes & open rivers. Have paddled in water in the low 40s, air temps in single digits. I would love to paddle as much as you do in a week!
Here is my "schedule"
Water temps 50 - 60 - farmer john 3/2 mm wetsuit. Add or subtract a drytop or neoprene shirt depending. Add a Polar Fleece (TM, by Malden Mills, U.S.A) underlayer depending. I prefer short sleeve underlayers and a short sleeve drytop, and longsleeved neoprene tops. NRS makes a Titanium series that is pricy but very versatile - great insulation yet flexy. I also own longsleeve neoprene shirts by BomberGear.
If not wearing drytop wear/carry a thin, waterproof & breathable longsleeved paddle jacket w. a slim hood, as a rain shield. Currently I like one made by Palm, of Event 150. Really great to have if you are racking up the boats, it's windy and you need a break from the windchill.
With a wetsuit I wear a 1 mm neoprene skullcap, 3/4 fingered gloves, & low profile NRS Desperado booties. Only fair to mention I have very long hair, the skull cap keeps it out of the way, so I nearly always wear one regardless of season.
Water temps below 50 I go w. a Kokatat Goretex Drysuit. Folks will go back n forth about the necessity of one, the cost, etc. Not here to debate that. I love mine, worth every penny, gives me at least 4 more months of paddling every year. Also after a day's worth of in the water classes, or a long day's paddle, I am much less fatigued.
Mine has the integrated booties. The joy of warm feet cannot be underestimated! Also a relief zip. Under this I wear different layers of wool, wool/capilene, polor fleece. My main piece is a wool/capilene union suit. On the outside I go w. a 2 mm skullcap, a polar fleece neckwrap, and the same low profile booties. I layer my hands as well: base layer seirus Thermolux gloves under either the 3/4 gloves or full neoprene gloves.
Over that go some Goretex pogies. I was very lucky to find some Stohlquest YellowJackets (no longer made) but I know Kokatat makes theirs in Tropos. I prefer these to neoprene pogies as they are not nearly as heavy (dry or wet), slide on easily (using one hand) and the Goretex really cuts the wind. Finally the light pogies come off swiftly when I really need them to.
You know of course you are dressing for immersion and that means water temps. However, I and others have experienced early hypothermia not from getting immersed, but just from getting a little wet and having a good wind on us all day, or even walking along shore back to the car. It all depends on the individual. It's just as hampering tho to your coordination and, maybe, depending on severity, your good judgement.
You asked for tips:
Whatever your set up is, test it beforehand by immersing yourself - like up to the neck. See if you can stay comfortable 15 minutes. Practice getting back in your boat. If all is well, you have the right set up. If not, get out of water, get warm, and add more protection. Repeat. It's good to have a friend on shore standing by.
Stay well hydrated - even tho it's very cold you are losing water.
Vaseline on the exposed areas of the face or neck helps, esp. if they do get wet.
Chapstick is a must! (and good for lubricating drysuit zips in a pinch).
Polarized sunglasses protect the eyes from drying out, which can be very uncomfortable.
Carry a hot nonalcoholic bev in an insulated mug.
If a full day, or multiday trip, a small drybag w. a backpacker stove, a small titanium pot & some dry noodles or similar makes a warming & sustaining break. The insulated mug also makes a good mixing bowl in which to add hot water.
Carry some little handwarmer packets in your PFD.The cool thing about the Seirus gloves is they have a pocket for these on the top of the hand, where they won't interfere w. paddling.
Severely cold weather is not the time to set new distance records, surf your largest waves ever, etc. Be conservative.
Paddling under a full neoprene skirt adds another layer of warmth. I do this year round anyways. I really like Snapdragon Supratex - warm, watertight, fit me & my boats very well.
Many will say do not paddle along in cold water - there is merit to that. I have, I would prefer company. It's up to you.
I mentioned the brand names not as any kind of endorsement (I'm not in the paddle industry) but as specific example of what I have tried that works for me. Doesn't mean that they are the unqualified best choices for everyone.
Happy winter's paddling!