Agree with your point about runners M&T into rails. This brings up a point about the order of assembly.
For a table with drawers, structurally similar to yours, I found it best working alone, to organize the assembly around 4 modules:
1. Lower web frame. Front and rear lower rails, with cross bearers M&T into rails, the cross pieces serving as drawer runners and support for vertical drawer dividers/guides. Rails terminating in double tenons for insertion into legs.
2. Upper web frame. Mirror image of lower web frame, except upper rails terminate in dovetails for joining to legs, and cross bearers serve as kickers and anchors for vertical drawer dividers/guides.
3. Leg modules, right and left. Front and rear leg (for each side), joined by panels M&T’d.
4. Top.
The back is its own module, with tenons to enter the rear legs. Must be inserted into leg modules same time as lower web frame. Then vertical dividers. Then upper web frame. Detail the drawer fit, and cap off with top.
If you mean glue up in modules, no, I glue everything at one time.
The web frame approach is a recommended way to proceed if that is the look one wants on the back of the table, back being opposite drawers. Not the look I chose for this piece.
A web with a dovetail end, another web double tenon and the panel mortised all perfectly inserted into a leg is a daunting challenge to accomplish without any gap at any of the shoulders. I did that once, but not thereafter.