In all my FM career saves I have boiled my personal tactics down to a GegenPress high press, high line defending with either 4-2-4 (as seen in the game) although for me it is more of a 4-2-2-2 or I use the tried and tested 4-2-3-1, I approach most matches with a Balanced mentality due to the nature of the Gegenpress now this tactic is most brutal on your players condition but does reap rewards when established and 9/10 if I choose a popular captain with high leadership skills and have most my players with high stamina 11+ and high work rates 11+ with a decent striker and pacey inside forwards/ inverted wingers, you will get goals and results, best form of defence is attack.
With tricky and harder opposition adopt at least a cautious mentality but try not to play too defensive as you will always be on the Backfoot and risk conceding more goals, peppered with shots, and likely to give away more fouls with the opposition being more attacking, save your defensive mentality for when your teams tiring or a good 2-0 up never settle for just 1-0.
9/10 with these styles together no matter what team I am I eventually have some sort of success, I won't lie with some lesser quality teams it is harder to adapt but once it does you will get the best out of all the key players. Let players who want to leave go as it's bad for dressing room morale and that never ends well for a team, always find out the players you can trust you will find these within the 1st half of a season and don't add too many signings unless absolutely necessary, I personally work around free transfers as if you sell them it's 100% profit.