Yes, some good comments here.
My 2 cents/pence.
Avoid buying scouts (since the new system) like the plague.
Power training is the way to go, but only when you have convinced yourself, he is a fast trainer/I want to keep him/ He has some good time remaining on contract. Power Training as Cat described is way faster on new system.
Avoid Personal Training like the plague.
I myself buy young very high 3's & 8's for a few reasons, less T wars, less likely that it is a more experienced player just flogging their 4's & 9's. But if you want Top end 5* wait till S4 for Negotiation. Now scout buys are useless, IMO best way if you are time poor.
I mistakenly bought 100 T mid way through S1 & S2. Not for 16 seasons and I survive with 5 & 6* now Nord or mainly from Negotiation and have a combined results of 972W-186D-219L.
These 'legends' with their 9* players have been "owned' when they play 35T league.
You have done some good math Southampton and have some sound advice above from highly respected members.
The law of diminishing returns definitely applies n T11.
Hope you do well.