Should be easy enough to fix support underneath the desktop and eliminate any issues with sag.
Or a beam underneath.
A supporting beam even.
Wickes do 20.5 x 69mm which I reckon would do the job, oriented vertically. Don't think that would interfere with your legs assuming regular chair and desk height.
@withered_preacher started
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Should be easy enough to fix support underneath the desktop and eliminate any issues with sag.