get a professional financial advisor to talk to your husband

Not a broker. He may listen to a professional more. Obviously he doesn't listen to you.
Paying off debt on current house is the worst thing to do. The interest rate is terribly low. And if you pay it off, you have even less to deduct for tax return. Any investment can do better than the interest you pay for current mortgage. The problem with no investment property is that once you don't work, you have no income. Other retirement plans (like 401k) will most time go down once you start withdrawing from it, while rent can always keep up with inflation.

The other option is to convince your husband to get a bigger house for primary residence (while keep current one for rental of course). If you have friends who are moving to bigger houses, he may be interested.

Seattle housing is no bubble if comparing to housing in NY, CA, or China/India. Location is good, and local economy is good.