Update on this, now that I did the dive. It was just a single 38 minute dive to shallow depths (I think 15.8M was my max), but I could have done another.
Wore 5mm gloves, 7mm hood, 3mm socks under 3mm boots. Main protection was a full 3mm under a full 5mm. the hood/gloves/boots were layered in-between the two wetsuits.
Dive conditions were 13°C at the surface and 12°C at depth.
Conclusion, for the single dive, I probably could have gotten away with the 5mm full suit alone, but the extra protection was nice, a 7m single could be nice if I move back up north (though a drysuit is much more likely).
As a side note, there was a LDS doing an OW class there (tomorrow is a 22°F dive for them, different location (spring lake)). They had much lower protection than I'd expect, I think one of the adults was in a 3mm, the others in 5mm. Most dove without gloves. The kids I feel bad for even more, one of them had to use a hot pack for their hands when they got out of the water.