One tool you can use is called a tenon cutter. It's the equivalent of the old-school pencil sharpener, a blade adjacent to a hole into which you insert the dowel and twist.
Amazon sells one that is designed to be used with a drill motor, but you could use it by clamping it and rotating the dowels by hand, although that would become tedious in short order.
The link above references a 3/4" cutter. There are other models which are adjustable for diameter of cut. Tenon cutters would represent an investment, as the prices I've found are in the hundred dollar range, but should last through the many tenons you'll be creating.
Another method, perhaps less expensive, is to have someone bore a hole in a thick piece of steel for the correct diameter. Dowels would be hammered into the hole, with the excess diameter being sheared/scraped from the dowel.
I did a search for "DIY tenon cutter" and found an interesting YouTube video, but it requires the use of a lathe to rotate the dowel at high speed. Inexpensive open end wrenches were ground to have a cutting edge on the end of one jaw, to be used as a form of lathe chisel. Not practical for basic tool owners, however.
More to the point is this YouTube video, which provides instruction to create one's own pencil-sharpener cutter, using a plane blade. The video describes creating a tapered tenon, but the angle can be adjusted to create a "normal" cylinder.
You suggest that someone could build a tool for you, but SE does not have the facility for such contacts.