Those floors look pretty good, I would probably skip going on the diagonal as that is usually done to flatten floors. But to answer your question I would sand the floor one diagonal and then the other and then straight to remove all the diagonal marks.

It does not look bad in the picture. But in person it had 1/4 inch variations in crowning

especially the area along the stair wall.

Ciranova EcoFix Plus ultra matte finish for no gloss.

Your first option is good. You do not need to go at a full 45 degree angle. 7 -10 degrees is all you need, unless the floors are very bad or the finish is super tough to sand off. The bigger angle is more aggressive, you will be removing more material, especially with a 24 grit. A 7-10 degree angle would be around a 6 to 10" pitch.

Sanding a cross-hatch "X" pattern will help flatten your floor. Be aware it is a more aggressive sanding pattern.

For a really flat floor, it's best to finish off with a three disc sander with metal hard plates. Rough the floors on a angle with the drum sander. No need to do a cross hatch pattern. Then finish off the medium and fine cuts with the three disc sander. And if, the equipment has the proper attachments, you can do a final sanding with a net type sanding disc on the same three disc sander for a scratch free sanding.