You’ve hit upon a good idea in my humble opinion. However, I also suspect that there are many other good ideas - and it would soon get to a point where if too many were implemented, the whole system would become overwhelming for the average user.
We’re in a similar line of work. I deliberately offer a three day turnaround for jobs that might take me just 20 minutes or an hour or two. This way I can choose to juggle jobs around should a larger order come in without impacting on the expectations of existing buyers, or else super serve my buyers by delivering quickly when I have the time. My point being that 99% of the time I’m in control.
This works for me because my buyers tend to be patient and prepared to wait for the result. I rarely get approached by people asking for a 24 hour delivery (which I mostly refuse as experience tells me these buyers are often the worst kind), and when I do have a request for a larger order that I can’t take on for another couple of days I respond with a custom offer with a delivery time of say 5 days. This often secures the work.