re: small-money bets that denialists can still get in on
Your simple measure won't work: warming affects precipitation which affects glacier mass. Measuring temperature directly through land stations, balloons, and satellites is better.
Arctic sea ice thickness isn't a bad measure though because it's dependent on air and water temperature, not so much on precipitation. It's only a regional measure, but that region is expected to be especially vulnerable to climate change.
I haven't looked it up but I doubt there's been constant increase in sea levels after the last ice age. I know some places like Scandanavia have seen decreases as the land continues slowly rebounding upward from having the weight of miles of ice removed.