Imo, the main responsibilities of the fde role are:

1. get the customer to the business outcome (if the product is insufficient, the fde builds what needs to be built to get the outcome, hence the engineer part)

2. take the learnings back into the product. the expected result of 1) and 2) repeating is your revenue and margin increasing and time to onboard decreasing. if your fdes are building from scratch repeatedly then you're going to be priced like a services business

most companies aren't willing to hire the technical talent that makes a good fde and aren't willing to run their R&D off of their fde team