This one of the aspects of the game that ppl have "well, it works" attitude. I personally had this attitude for long time, but Advisers system can be further improved.
If you played this game well enough, you would quickly learn that this game have three types of Advisors.
Obviously good ones or must buy ones, 10% morale, 5% discipline, holly duo of production/trade efficiency.
Situational, when you need that extra Diplomatic reputation, Colonial range, Missionary strength etc.
And the last one, the cheapest guy on the list when you just want the monarch points.
That is not amazing system but it works.
There is also invisible for majority problem, is that AI gets Advisors basically on random. There is probably math formula based on cost vs how Valuable is bonus from Advisors. But generally it doesn't work. The only way to fix it via modding currently, is to disable "weak" types of Advisors for AI.
So my suggestion is to remove Advisors with "weak" modifiers from the Advisor pool. Their Bonuses will never be valuable on their own, but they can be repurposed as Advisor perks.
Perks can be tied to how long Advisor have been in service, so it will appear like natural career growth, or experience gain. Advisor level would just increase experience gain.
Perks acquisition can be done in different ways, preselected for Adviser type, semi random from preselected pool, or just player choosing the perks from a preselected pool.
The main goal here is to make every Advisor type at least Good/Situational. And perk system gives some room for flexible combinations, for example you take +10% Manpower Advisor and over time he gets you additional +10% Morale bonus.
Level 2,3 Advisors in the pool are just redundant. As you can always upgrade them, and perks can increase Advisors upkeep even more. AI struggles with current formula, it would be much easier to set priorities for AI with only lvl 1 on the list. Programing logic behind AI upgrading Advisors seems like it would be a straightforward task. But it's just my slightly educated guess.
I would also suggest introducing drawback for having Advisors that stay close to power for so long. Extra cost, possibly tiny amount of corruption. Corruption generation from Advisors sounds like logical step, "power corrupts" etc. But with current corruption implementation it's hard to justify adding plus corruption modifiers. It's just to strong and mainly affected by money spent on routing corruption.
From: https://forum.paradoxplaza.com/forum/threads/advisors-rework.1275257/