If your wetsuit fits like a glove and allows minimum water to enter and minimum water to continually flush then you've got a good wetsuit that will do it's job.
I don't know what the norm is for wetsuit divers in Lake Michigan, but here in Monterey serious wet-divers buy custom-fitted 10mm farmer john wetsuits w/ attached hoods and custom fitted no-zip booties.
Ways to improve am existing wetsuit are buying a hooded vest, buying a hood, or taking it to a shop that will do altercations to fit it to your body measurements.
You could also buy custom fitted hooded vests, booties, hoods, or a whole wetsuit setup from said custom wetsuit shops.
Generally the custom-fitted exposure protection will provide better warmth than stock items. Other custom-fit variations that provide more warmth are
- no-zip booties
- attached-hooded vests/jackets
- skin-in neoprene
I wouldn't recommend wearing two wetsuits unless you're on vacation, in the middle of a temp gap for your two wetsuits, and don't want to buy the appropriate exposure protection.
Doing so can stretch your suit, allows more water to enter and flush through, and you generally don't gain as much warmth compared to if you bought good fitting exposure protection in the first place.
And even if you do pursue that route I only recommend wearing a farmer john jacket as the outer layer, since you don't have to combine neck seals of two wetsuits then.
In your specific case, 2.5mm is not sufficient enough to keep out the cold of Lake Michigan. If your 7mm flushes with water, you'll only have the 2.5mm to help you out. Think about how your neck's going to feel with a hood and two wetsuits around it as well.
For home diving use, spend your big bucks on good exposure protection. This doesn't always mean dry suit btw.