Best cloud alternative to a Mac AFP onsite file system

If all they want is simple on-prem shared storage, then a couple NAS boxes could work. With that many people, they should have at least one dedicated person dedicated to managing things, ensuring backups are working, etc.... Configure one as the storage and use the other as a backup. Synology is a good place to start looking. Another economical backup option is Backblaze.

They are big enough that they really should be looking at something like Google Workspaces or Microsoft 365 which will support the security and management features that are necessary in a larger environment. I think they can afford the ~$200/mo. cost and will get a heck of a lot more for their money.