Friendly order number - start sequence at specific number
Once you have created the sequence (starting at 1) with your sequence key/id you can manually reset/update the number in the craft_sequences table in the next column. Obviously if you reset the number, you could get duplicate order reference numbers though if you are not careful.

Alternatively, you could just let the sequence start at 1 and use twig to add 11000 to the sequence number within the Order Reference Number Format.

Your Order Reference Number Format would look like this: 11000+seq('mySeqId')