Ignoring financial info, any opinions either way on using sub-projects? Basically only use them to track short intervals of work with various work-specific data and resources (time entry by resources would be required).

The generic reply here is please do not add levels to anything unless it is absolutely needed. it makes reporting etc. a pain (there may be exceptions, never used sub-projects.)

Appreciate the sentiment. Our use case would be a sales order line item that’s basically a drawdown with incremental fulfillments as needed. I was thinking we could have the parent project be on that line item, then sub-projects created as needed for x days of work then fulfill x qty on the line item. thoughts or different approach?

No that sounds ok as it keeps the 'untangled' which I think is what you want (so you do not have to change an approved SO, SOX compliance etc.)