Sharing folders between VMs on ESXI 6.5 for backups

Using a folder on a VMFS volume is nothing an ESXi host supports - a VM guest cannot ever access a VMFS volume.[1]

A VMDK is a block device like a disk drive. Sharing it across VMs requires a clustering file system with coordinated access. Using a normal file system will quickly corrupt the volume due to concurrent, uncoordinated access.

Of course, you can assign a VMDK to a guest and then use network file sharing (NFS, SMB, ...) for other guests to access. If the share doesn't provide enough performance you need to speed up the network (or the host, for a virtual network).

[1] That is unless you use the appropriate APIs, SFTP or such.