Can I save e node with 6 critical failure (file not found) or just delete and start over?
If your node would be disqualified (please, check your dashboard), then you can still continue unless your node is disqualified on all satellites.
If it’s disqualified on all satellites, you may start over.
However, it’s better to figure out why your node lost files to do not repeat the same mistake for a new node.
My mistake during migration. Hope to see in the future a way to fix so few errors (4 critical error on 5tb 6 months old node)
If the mistake is just 6 (or 4) missing files from 5TB then as @Alexey said you should continue to run the node. The disqualification level is 96%, so your node should well be within tolerance.