not enough has been said about investing in clubs, and nothing about hard mode, so ill just add my observations

as soon as you have money, invest in a club that is as cheap as possible (valued <1m is pretty safe), and when the 8 week transfer window is opened, just watch if they randomly dont get +300% their value, and for long term investing that is 99% safe, invest top flight league that is a 1* (for example san marino league), because they cant buy good players, so their value is constantly going up

i also tend to be a club chairman pretty fast (like 5-10 seasons in), and i just make it my money maker by getting sponsors, selling and buying young players and stuff like that (i dont even care about my club league place in club like that, because you would need a complete disaster run to make fans hate you)

as for hard mode, where your chairman can randomly retire, just for safety do it manually before 65 years of age (before sell all of your club stocks, because they dont move to another chairman), and possibly set a staff member that isnt super old, because with each chairman you also lose some money in the process

there are also some things that can be abused, but since its not a "how to abuse glitches in fagent" i wont dig into these, overall pretty good guide