Bisquit cutter makes it easy. If I were going to use a slot cutter, I'd make a 45 degree jig to clamp against the table so I could slide the frame toward the cutter, using the edge of the fence opening as a stop.
On my frames, I usually assemble first, then put the frame into an L-shaped spline jig, and use a 1/8th inch, flat tooth blade to cut a slot for the spline. My splines are generally some sort of exotic wood that makes a nice contrast. I often use purplewood. I also do a lap miter from time to time, which gives you about the same glue surface as a biscuit but also provides for a rebate for the canvas stretcher.
Biscuits make a very clean, modern looking frame. That's a nice looking piece of work.
The more I do, the less I accomplish.