As The OP says he has a vertical mill might as well stick to methods that suit it’s use.
This is one of the methods I mentioned earlier if you don’t have much in the way of tooling. I’ve assumed a vice and a milling cutter are available but not much else.
Pack the work up on whatever you have so it stands just over 1mm above the top of teh vice jaw. Touch the cutter onto the top of the rod, note height then reposition cutter 1mm below the crest of the rod and take a cut.
Loosen vice, rotate stock 90deg so face just machined is against the fixed jaw and cut again. Repeat & Repeat again.
I saw this method in a book aimed at beginners😉
Add a length stop if you want or if the edge of the cutter is set flush with the end of the vice jaw as I did here then you can just push the shoulder from the first cut against the vice jaw.
Note if a shorter length of rod then stop the machine when repositioning the work.