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Do not build this — and check first whether you should be paying at all, because this is a market with unusually good free options.

Sync clients on every platform, block-level transfer so a one-line change to a large file moves kilobytes, and infrastructure that has not lost a file: that is the product, and it is mature.

WHAT TO DO INSTEAD
- **Syncthing**, if the requirement is your own devices in step. Free, no server, no account, no company, and it does the job
- Whatever cloud storage came with something you already pay for, which is often several times what a standalone subscription gives you
- Keep it only if you genuinely need link sharing with people outside your network, or the specific integrations

WHAT YOU CAN BUILD, AND SHOULD

**A verified backup**, because sync is not backup and this is the confusion that loses data:
- Sync propagates deletion. A file deleted or corrupted on one machine is deleted or corrupted everywhere, immediately, and version history has a limit
- So: a scheduled copy to somewhere the sync cannot reach, with its own retention, versioned
- restic or borg for the backup itself, encrypted, to an object store. Do not write a backup tool
- A verification job that restores a random sample and compares hashes. An untested backup is not a backup, and this job is the whole point
- A monitor on the age of the last successful backup, alerting somewhere you will see it

AND THE THING WORTH KNOWING
Sharing a large file with somebody outside your network is a different problem from syncing your own machines, and it is genuinely buildable — expiring links, resumable uploads, real deletion. That is covered under WeTransfer elsewhere in this catalogue, and it is a fortnight.

THE ONE-LINE VERSION
Use Syncthing for your own devices, keep a real backup somewhere sync cannot reach, and test the restore.