I’d say you improve by developing and strengthening your starting XI and squad to be honest. It’s not about specific individuals.You improve by selling the players that aren't good enough, not by selling your best players. I will wait till the end of the transfer window to pass judgment.
At our level if you want to compete with those above who have significantly more resources than us it’s also about coaching players to improve them, not just trying to buy ready made players either.
If we sell a player and replace them with someone who isn’t yet better but we can develop to be then that’s good business and a smart development strategy. You just have to get it right.