because spaces endpoints (currently) exist only within the public ip space i think it won’t matter whether you have a private ip or not because Spaces outbound is public only. i’d like to see private endpoints but have no idea if that’s planned. for what it’s worth, below may address the root concern you may have here.

there are no bandwidth charges to/from Spaces/Droplets so long as the the Spaces objects and Droplets exist within the same regional datacenter.

see: https://www.digitalocean.com/community/questions/spaces-bandwidth-pricing

no answer yet (at the above link) as to whether or not both Spaces & Droplet must be owned by the same user account.