Yeah that's the thing, and for a U21 where you got a time limit (21yo), if you want secondary skills you got to train them first.
Let's say X is 2 in your example.
Player 1 gets only outside training:
year 1 - 10 OSP pops
year 2 - 8 OSP pops
year 3 - 6 OSP pops
year 4 - 5 OSP pops
total : 29 OSP pops
Player 2 gets 1 year inside training then 3 year outside training:
year 1 - 5 ISP pops
year 2 - 8 OSP pops
year 3 - 6 OSP pops
year 4 - 5 OSP pops
total : 19 OSP / 5 ISP
Player 3 gets 1 year inside training at 21 after 3 years outside training:
year 1 - 10 OSP pops
year 2 - 8 OSP pops
year 3 - 6 OSP pops
year 4 - 2.5 ISP pops
total : 24 OSP / 2.5 ISP
Player 3 is way worse than player 1 in primaries, and only marginally better in secondaries. If you want secondaries, player 2 is the way to go.
For NT the reasoning is slightly different. In the end you can get the same number of pops, but if you focus on primaries, his salary will go up way faster, and you risk capping him before you got time to train him on secondaries. The other "problem" is that when he's 25, you'll see him pop maybe once every 6-8 weeks if training out of position, instead of once every 3-5 weeks if training primaries. The elastic effect helps getting these last few pops quicker though, you just got make sure to not hit the cap