Storage Spaces Direct very low available space

I vote for shared storage. For the case, StarWind Virtual SAN Free seems to be a better option. Microsoft Storage Spaces Direct are more about Enterprise having requirements to underlying storage and Windows Server's licenses. https://technet.microsoft.com/en-us/windows-server-docs/storage/storage-spaces/choosing-drives

From my perspective, StarWind Virtual SAN would work here better since does not have mandatory requirements to underlying storage and provide shared storage on top of underlying with less to none penalty to both performance and capacity. https://www.starwindsoftware.com/starwind-virtual-san-free

S2D needs some free spaces to self-heal it in case of a disaster.

https://blogs.technet.microsoft.com/filecab/2016/11/21/deep-dive-pool-in-spaces-direct/