I guess the thing you need to think about is what are you hoping to achieve.
Basically you have a strong horizontal platform linking two multi box structures (the rear body with engine and the front with the frunk etc). The two box structures have multiple bulkheads and horizontal structures with notable vertical sections.
The center platform has few of these things. It is basically two outer beams linking the two boxes at the bottom and a center beam with a little more section but many openings which compromise its integrity. Between these beams is the floor which really is along or the ride relative to the beam strength of the center of the car which is why one can cut the floor out with relative impunity. The floor does have torsional value and due some of its structuring, load transfer across the center of the car in side impact.
OK so ultimately to get the ultimate in structure you should weld the doors shut. This would join the front and rear boxes most efficiently. Clearly that is not what you would want
but it does demonstrate that you want to get the bracketry added to be as tall as possible and reach along the horizontal beams as far as possible.
As the side beams are attaching to the two boxes at the bottom, the best thing to do to is to improve the connection of the beam to the box. The taller that connection can be, the better. You have already done some of that. I would posit that you might consider making L shaped braces that run further along the horizontal beam and then up the B pillar further. The thickness of the L can only be as tall as from the floor where the seat mounts to the top of the beam and preferable across the top of the beam. It would connect to both the floor, the vertical face of the beam and the top of the beam. On the vertical leg going up the B pillar it could be deeper than the existing structure because your seat is always going to be in the door opening anyway, if there was some steel alongside it there really woudn’t be an issue in terms of entry or egress. This would want to be boxed as much as possible so for example it could be as deep as the mount for the seat belt reel and run over and be welded to the weather strip rib or have a flat with plug welds to weld on the flat alongside the seal rib. There is a limit to what you will gain by just adding more material following an existing section versus adding more vertical section meaning making the structure taller. A thick walled tube is very little weaker than a solid bar of the same diameter due to the dimiinsisting I value as you get closer and closer to the.
Honestly making a super stiff connection from the flexible flat floor to the flexible back bulkhead in front of the gas tank is not going to achieve too much. The action of the structure is happening out by the doors not inside the floor pan. What you are showing for the structure in your hand drawn pictures won’t help much to achieve what you need.
One thing I didn’t mention is adding more section by going downward but I don’t think that is possible due to your external body work.
My apologies for being a downer.