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This^You can still do those mental toughness sessions once per week or, ideally, much less often like once every couple months. If you track percentage of improvement of a benchmark session (ie., Fran or PFT) you'll likely see greater percentage of improvement which would be more normalized among different fitness levels of different individuals.
I am a big proponent of including higher metabolic sessions as a regular rotation, but as with many attributes, hitting it harder all the time doesn't guarantee gains. The effects if done consistently are cumulative, training metcons all the time detracts from other equally important adaptations.
It doesn't take a ton to improve or maintain. I try to do a metcon session every week, but its the first thing I cut if I have to skip a day/shift sessions around.