Since Seeds are designed to be an alternative to 100 Mario Challenge, why not expand the max size to 16 since 100 Mario Expert is 16 courses long? Any time I’ve seen it raced, it was on Expert difficulty, so I feel it could aid in making those fair too.
The seed feature is quite heavy job for the servers to run, so upon release we decided that 10 was a good number to cap it at for now, until we have more data. It might increase in future. 
Personally I think 10 or less levels is a better fit for racing. Instead of one long race you could do multiple shorter ones, with the possibility to take breaks between. But that’s my opinion.