I understand what you are saying, but unless someone is downloading an extremely large amount of data for a very long period of time, any temporary slowdowns in network speed would probably go unnoticed.Hold on a sec, you _might_ make good use of that 1GBps if you have a lot of 500MBps devices all downloading at the same time.
Not saying it never happens, but how often to most home networks have over 500Mbps downloading?
Streaming is the biggest draw of bandwidth for most people, and 500Mbps would be overkill for streaming even for a very large family.
The OP still hasn't stated what his family's internet usage looks like, but if it is mainly streaming, even as low as 100Mbps internet service would probably be fine without any noticeable slowdowns.