GeorgeFloydOverdosed
I observed in the spreadsheets that set pieces decline -1.00 even when you have CA growth up to 15 with the blue 'general' training sessions. By contrast there is 0 decline when you use many of the other session types.

So I came up with some theories.

The first was that perhaps instead of set pieces & technicals in general being squeezed out by low CA, it was instead the 'general' sessions being usually 80-100% priority while the other sessions are 20-60% priority - perhaps the the remaining 40-80% was directed to a hidden pool of attribute growth, just like how 'rest' increases physicals by default. This hidden attribute growth would be significantly more overall growth, which makes sense, as in games focusing on specific attributes often carries an efficiency cost as a game balance.

Perhaps 'priority' can be stacked up to 100%, but anything more simply changes the proportions of that 100%, and each extra session is an extra session of hidden attribute growth. So for instance, 'attacking' (80%) + 'overall' (100%) = 2 x hidden growth and 100% directed growth, whereas 'goalkeeping' (20%) + 'penalties' (20%) + 'outfield' (60%) = 3 x hidden growth and 100% directed growth. This would make sense from what we know SI does, where they use misdirection to keep the true mechanics unknown - in this case, the best training regime would be what appears the least likely and simplistic.

But I did some rough tests, and can rule all of the above out. HarvestGreen22's Quick + Attack + Rest + Quick focus produced significantly better results with less injuries.

I thought I'd share some screenshots, as this shows what you can expect in reality as opposed to a perfect environment averaged 1000 times - one season, England Div 3, 3-4 star coaches, a generous amount of good but not amazing youngsters actually training at the club, just let the chips fall where they may in terms match experience, injuries and so forth (but I'm still selecting amongst the 'survivors' to show here).

Walters_quickattack = Quick + Attack + Rest + Quick focus. 40 injuries.
Walters_mixed = My dense but optimized regime based on theories above. 80 injuries.
Walters_extreme = Complete overload on physicals training + 2 x match practice & 'overall' to boost technicals/mentals. 207 injuries.

So in typical play is the HarvestGreen22 training significantly better? Yes, it gives 1-2 extra Pace/Acc in 1 season, halves the injury rate, and might even have an edge in mentals over a more 'balanced' regime