: If this is one of those topics already covered,

Yes, covered before.

: I like to keep my
: boats as light as possible without becoming fragile.

Use 3 mm ply. Don't use any glass except on the joins. Along the inside and outside chine and keel join. You don't really need it on the deck/side join. Glass soaks up about 2- 3 times as much epoxy as straight on wood. Every litre of epoxy weighs 1 kg (over 2 lbs). I probably use less than 5 litres on a kayak, that would mean 15 kg of epoxy if glassed. My latest hull (launched and tested) was 13 kg before fittings (rudder, pedals, hatch covers, decklines etc.).

Life? Probably 30+ years. My oldest kayak was built in 1983.

: I am
: careful in construction, use, and transport of my boats.

Add another couple of decades to the life then.

: boat will be used on Lake Champlain, for recreational paddling,
: no expeditions or big coastal waters.

Add another decade.

Our kayaks do multiday trips, get bashed on rocky beaches and rocks at sea, hauled around the country for thousands of miles, refitted only every 5 years or more (bit of paint or varnish). The previous kayak has done a decade of paddling and has been replaced as the main kayak because the latest is a little more stable and carries a little more and I needed a new design. The old one still gets used and the oldest sometimes if there aren't any other available.

Alex