It's a little complicated with expiration. Your miles expire 36 months after being earned unless you are an elite member. In this case, they don't expire. So, it is worth going for elite status if you have soon-to-be-expiring miles. Or, just earn and burn! I don't recommend keeping large mileage balances for long periods of time anyway. You will miss out.
With partner awards, you need "low" mileage availability to book. However, it's actually more complicated than this because Delta appears to have special "low" inventory for partner awards. You may find slightly more availability using a partner to book on Delta at "low" level awards versus the SkyMiles program. Still, the most valuable redemption is roundtrip to Iceland from North America and there is plenty of "low" availability regardless, because this is a seasonal service that doesn't seem to be widely known.