Once you have a plow plane, your life will get simpler. The plow plane features a fence to guide it along its path. The fence is adjustable, some by wooden screws, some by captured wedges. The iron I use is only 3/16″ wide. I line this groove up in the midst of the 5/16″ joints I cut. I start this plane at the far end of my stock and work backwards as I work forwards.
Light shavings and sharp irons are key. I make the groove about 1/2″ deep. I extend it beyond the mortises so it reaches full depth throughout.
Some panels are raised. I use beveled panels. Trim their rear faces to a long, sloping bevel to feather the edge to fit the grooves.
Beveling the panel is easy. This isn’t raised-panel territory. The inside face is beveled and the outer face is flat (mine are usually carved). I use a hatchet to rough out the long shallow bevel. You can substitute a scrub plane and/or a drawknife if hewing is too daunting. I follow the hatchet with a smooth plane. My bevels fair down almost to nothing to fit the narrow grooves.
A test assembly is imperative; adjustments are made. Once the frame will close up around the panel, pin the joints. In my world, mortise-and-tenon joints must be drawbored. Once you can make the frame and panel, nothing can stop you. Go.