Flushing on subsites?

Yes, check out the WP_REDIS_SELECTIVE_FLUSH option.

https://github.com/tillkruss/redis-cache

I have that option enabled but do not see a menu item inside admin.

It happens automatically when flushing individual sites. If you need better control over what’s flushed, please submit a pull request here: https://github.com/tillkruss/redis-cache