Gonna be good !! - that front rail (and the front to back ones for that matter) aren’t doing a whole lot in terms of limiting flex - I would think . But if you glue and screw the front rail to your desk-top from the underside (pocket hole jig would be ideal - or little tiny metal corner brackets) then it’s stiffness is acting on the front edge of the desktop as a beam - to limit flex. I did this over 1700 mm - doesn’t flex even with the weight of a 10yo . So you might even get away without a middle leg.
A while ago I made also a big desk (~3,somehting meters wide), and I built it too low, so I unmounted everything and put the desk higher with some wood under the table. The depth of the table is now 67 cm.
Gonna be good !! - that front rail (and the front to back ones for that matter) aren’t doing a whole lot in terms of limiting flex - I would think . But if you glue and screw the front rail to your desk-top from the underside (pocket hole jig would be ideal - or little tiny metal corner brackets) then it’s stiffness is acting on the front edge of the desktop as a beam - to limit flex. I did this over 1700 mm - doesn’t flex even with the weight of a 10yo . So you might even get away without a middle leg.