[Answer]-Custom field / form field (maybe widget?) for storing values for a string format

This may not exact answer, but writing it in comment is not feasible.

Another option would to store as JSON string in the model. Like

[{ "Pattern": 'Strength deals %d more damage for each %f%% critical chance',
  "Values" : [2, 1.50]
}]

Use custom form field to input/output as JSON input. There are quite a few implementation available when searched on google.

With this approach, you can try to validate the inputs, so that format string and number of variables provided matches.