Slot Cutting

Same as you plus a slot cutting router bit.

Same as above, and
A grooving plane plus a chisel when I am feeling neanderthal.

I usually do the first. Either will work. However, the latter requires proper alignment of the same cut on two machines. if that is important to the final product then it make it more difficult. In either case, I'm not sure what the advantage would be using the latter.

A dado blade on the table saw, a slot cutting bit on the router with or without a table…..both would need chiseling if you wanted a squared up the end.

Shaper with router spindle or a Horizontal Mortiser, Palm Planner with a Sled.

Depending material size I use my Milling Machine, if not same as you.

I prefer the second method. Drill press and router table. Drill press and fence to get through holes in the exact beginning and ending of the slot. Register the router bit with fence and 2 stop blocks. Raise bit to take multiple gradual passes until bit makes it to the top. I don't like lowering a piece of wood onto a spinning bit like the first method.

Through slots are almost always done on the TS with a FTG rip blade. Minimum wear and tear on the cutters, easy to center and adjust the slot width.

For stopped slots, single pieces, I'll drop down on the router bit (table) and do as you describe with start/stop marks on the fence or stop blocks clamped on.

If doing many pieces and if the final project would accept the slight grain discontinuity, I'll use the TS method first mentioned, then insert fill strips into the slot to cap off the ends of the cuts.