Don't expect a fix for this.
Every FM version has been the same. A lot of players prefer short games, one or two years before starting a new one.
Now, with the increase in FM addicts population, the world simulation has been further tested. This has arisen all the thread's questions.
So, to simplify the matter I will try to write what to expect based on my own experience of the game.
1. All (non-custom) playable leagues will hire managers and will have a full line-up. But, if you load a very low level custom league, like division 6 in any given country, you may have a few teams with grey players due to low reputation numbers in clubs.
2. Custom playable leagues will depend on the database you load. If you don't use a players' database, the game will fill clubs with mostly bad newgens, so you will have to wait some years for that league and clubs to be competitive. But they will have a complete line-up.
3. Non-Playable (non-custom) leagues will start with their full line-ups if you configure that option through advance database, but in time they will gradually lose all or most of their players, especially clubs in mid-low tier countries. And they will not hire any manager after sacking their former. This is the main problem of this thread. This disrupts the feeling and simulation of the game for some of us.
So, one year, say 2025, you will be playing against a good swedish club, like Malmo, on Champions League group stage, and you will notice that they don't have a manager, or their line-up is full of grey players. And then you will start wondering, searching, hoping this is only a Malmo bug, and every other club have at least a manager. And no. They don't. The Europa League winner doesn't have one. You even notice their goalkeeper is grey, and that's only the beginning, because if their goalkeeper has gone grey, then all their friends will soon follow. Euro Cup II winner and runner-up are a lost cause. No manager, at least six grey players each. They won't ever have a chance of succeding in any continental competition again. EVER.
Except maybe Europe League II, Asian Champions League, African Champions League, World Club Cup, Copa Sudamericana, etc, where you will consistently see two managerless half-grey clubs battling on the finals.
And what's left? Only loaded leagues will start dominating competitions. No more surprises. You loaded Germany, France, England and Spain. That is the limit of your world. It is not alive: it is restrained. Those countries will always be on top. Boring games until you reach Champions leagues' Quarter Finals.
You will lie to yourself saying that only your league matters, but that is not true, because the heart of a simulation is the possibility of being surprised by it. And that cannot be limited to your own league. Because knowing that you will ALWAYS see the same four countries winning at continental tournaments is enough to, at least, obscure the experience.
4. Non-Playable (custom). This is hell. If you didn't add players with the custom file, you will get full grey-player and managerless clubs from the beginning. Only clubs with very high reputation (6000+) will hire some (4 or 5) players through the game.
A piece of advice:
If you are like me and want a more alive, fluid world, but you don't have a great CPU, then you'll have to give up on details. Download FMT and load all possible leagues. You won't have all Full FM features, but your game will play fast and the simulation will be a lot better.
My downside: I miss you, Japan.