Zoho Flow doesn't currently have the ability to process loops with low code so you would still need to write a deluge function within Zoho Flow to loop through the subform records.
Because of this I'd suggest not using Zoho Flow and using a deluge function associated with a workflow to retrieve the subform data (Only 2 additional lines of code) and then loop through it to create your records based on the Subform Data.
The best answer might be a mix of both, I personally prefer using Zoho Flow, to keep everything in the same place and as low code as possible. But, either way as Hugh said you will have to write some code, I find this easier in Zoho Flow as once you write a function you can save it and use it again with different variables in the future (This is just personal preference, everyone is different). If your okay writing deluge, you could extract the subform entries and do all the calculations in Zoho CRM through a custom function, in my opinion however this is harder to maintain and less accessible for normal/new people who have not learnt deluge.
I appreciate the insight though and a hybrid approach might be a good start, you could have the custom function in Zoho CRM extract the subform payloads like in Hugh's post and then send each entry as an individual webhook payload to Zoho Flow where you can then use low code to finish the process.
In summary:
Hugh's suggestion = most efficient, most difficult (if you don't know deluge)
Max's suggestion = Low code, less difficult, not ideal for high volume, still requires code.
Hybrid approach = Less code + efficient, medium difficulty since the function is just for extracting the entries and sending them as webhook payloads.
You are so close but you need to populate the createMap with data prior to creating the record and you need to extract the field values from the subformRow object relative to the subform fields using their API Names.
You can find them under Settings > Developer Hub > API & SDKs > Accounts > Right Down the bottom you will see the Subform API Field Names.