To climb leagues? Just see what are the best built players for your specific division and buy the most impactful ones and salary efficient, SB bigs are the rage now, and I used them in my back to back promotion from III. to National League.
Other than that, you have to take advantage of every single thing you can to get over your opponents, 22-0 is automatic promotion(almost always) so you can try to attack the promotion by achieving that, or you can try to finish in top 4, make some money during the first 10 weeks, and then go all in before play-offs with salaries higher than your highest competitor.
Training Game Shape for the season is a good idea to always guarantee your players are in tip top shape, sports psychologist to help mantain your GS even when you overdo minutes or underdo it.
To win National League you will need a bit of a different plan, but until you get to National League, it should be very achievable to invest wisely something like 5M in your team, and have the best team in the league. Analyse the other teams and see who is spending on which players and how much.