The first few sets I made using a 3hp Makita Router and ½” flush trim bits. Although I built several sets, it didn’t work great. The bits burned and didn’t last long. I tried a 3hp Ryobi with a speed control and slowed it down a bit, but then the router didn’t have the power to cut the notches in one pass. Then I found a 3/8” flush trim bit and that worked quite a bit better still. Eventually, someone suggested a shaper and that worked best of all. I learned a 1 hp shaper is more powerful (a lot) than a 3hp router. The bit stays a lot cooler too, because removing the chips cools the bit. I could never figure a good way to collect the chips when using a router, and I tried, but its easy with a shaper.
Here is the pattern on top of the blank board:
You can also use a table saw to cut the notches. Here is an example: https://www.youtube.com/watch?v=JiZk1AsHIto
He moves a lot quicker than with the shaper, but I like the fact the shaper rounds all the notch corners and board ends (assuming your pattern is rounded) so there is less sanding. And with the table saw its another step to cut the four corners from each board.
With a pattern, when you go back months later to make more pieces, they will exactly match the earlier pieces. Since all the boards are very, very close to the same size you can gang the boards (with the same number of notches) up together, and sand the edges.
I use a flap wheel sander to sand between coats. With a spacer between two boards I can quickly sand the edges of two notched boards at one time.
Cutting notches moves pretty quickly now. It takes about a minute to cut a 2 notch piece, two minutes for the 4 and 6 notch pieces and 2-3 minutes for the 8 and 10 notch pieces. If everything goes well I can cut all the notches is less than a day.