A little on the costly side for one job, but if you have access to borrow....
I have a Dymo Rhino label maker from my home theater days. It only handles ½” wide label tape. I prefer the one we have at work which will handle the ¾” tape.
How many letters you can fit in the width of the tape is determined by the font size you select.
I added fuel injection, fuel pump, trunk battery, turn signals separate from brake lights, headlight relays, etc., so I needed more than just factory diagrams and colors.
I’ve used both sizes in my car, ¾” in places where it won’t be really visible, but ½” in places where I want to be more discreet. With less text available, I have to be a little cryptic with abbreviations sometimes.
Rather than flag type tags, I set it to vertical wire wrap. After I put the label on, I tried to use clear heat shrink over the label, until I ran out of clear shrink and didn’t feel like waiting to finish. To heat shrink, you have to be putting new terminals on as well, (not just labelling the old wires)
If you don’t want to see the label later, you can still overwrap the whole harness and unwrap a few inches of harness if you ever need to find the labels, then re-wrap it.