u/Ball6945

I have been building this table mostly by hand and decided to challenge myself with joinery(first time ever building a table in general as well)

I have most of my joints cut out and the overall thing fits pretty well. I wanted advice on how to finish this last portion as I noticed it likes to wiggle side to side. I figure it would go away if I do another set of through mortise and tenons that go lengthwise on the bottom OR should I add a diagonal brace from a top corner to the bottom?

What would be the best? This will be a table used for knife making in the future, 200-250lb belt sander and frequent hammering as well as back and forth rocking motions.

Thank you very much