Sue
For many years our church (Westside Unitarian Universalists) rented space at the Alki Masonic Temple near Alaska/40th. I don’t know if they ever secured another weekly rental on Sundays (we moved 2 years ago), but there was these service room, then we rented the downstairs hall for our coffee hours and childrens programming, as well as an upstairs we rented for offices/classrooms. It would probably be too cramped for 250 (we struggled with the space for a childrens program), but might be worth calling to inquire.
Back in NYC we also had a lot of churches that shared buildings, where the home church had Sunday morning services and the rental church had them on Sunday evenings. If you’re not wedded to the idea of Sunday morning services, that could be an option.