To be honest, you don't need to do much to the squad in the first season.
Go with the 'Geigenpress' mode when you select a formation in tactics and a 4-1-2-3 in attack philosophy worked wonders for me.
Sweeper Keeper on attack, Full backs on support, Two Center-backs on defend, a Defensive midfielder sitting just behind the two midfielders on defend, a box-to-box/central midfielder/carrillo on support (depending on if you are playing Jack/Kamara/Arfield in the position respectively), attacking playmaker on attack, on the left wing have Kent as an inside forward on support, on the right a winger on attack and a pressing forward up top.
A defensive midfielder on loan is a must. After the first season, Ross McCrorie and Jack are incredible in those areas.
McGregor retired after the first season in my game but Robby McCrorie develops into a very decent goalkeeper and Bailey Peacock-Farrell is a steal from Burnley because they have a tonne of goalkeepers.
Aaron Hickey and Harry Cochrane can be picked up for about a half a million total and they turn out good. Lewis Ferguson is also a tremendous midfielder in the game and Carlos Alena can be picked up from Barcelona on Loan.
I picked up Shankland as a back-up striker for Morelos and it worked out well. However, if you sell Morelos and want someone like him, you can Pick up Felipe Caicedo from Lazio for a couple of million as you wait for Richie-Hosler to become a mega-star.
Alex Lowry in the midfield for the Under-18's is a brilliant deep-lying playmaker by the second or third season too