Kosta has only played about 12 times this season in the Serbian League due to injury, he will need time to get used to the prem. Rogers stepped up from the championship which is superior to the Serbian league. It's also alot easier to throw an attacker/midfielder in at the deep end as opposed to a defender because mistakes made at the back will more likely lead to a goal.
Konsa's best position is CB alongside Pau, he should only be played there for rotation purposes or due to injuries. I don't think KKH will make it, multiple managers have run the rule over him and none have given him a chance.
Digne has been great this season but will only decline due to his age if we can upgrade in the summer with a younger alternative we should. Moreno has been poor compared to last season, his injury at anfield really seems to have had an impact.
Digne's destination will probably be Saudi, they don't care about book values.
The rest of the players you mentioned are development players and not ready for prem football.