I've fought the gate sag thing for ages.
be aware, the corner-to-corner cable and wood approach puts the sagging gate into torsion aka "twisting"
eventually the gate becomes no longer "flat"
if you can get the cable to pull "dead center" of the structure, that works. but achieving that is not so easy.
if you have the option, install something on the latch side that supports the weigh of the gate when closed.
it may sag a bit when opened, but if it spends most of its time closed it will not sag&deform.
a taper lead on a block on the latch post, if the ground falls away on opening a extension to the latch stile, there are "spring loaded roller" gadgets available....
really depends on the individual situation.