My usual disclaimer: There is no one way to play, but all my advice is based on how I play.
1) Even with surgery at 8+2 I take tons of losses...
I consider surgery 10+4 an absolute must. With qualis gems, there is little reason not to. I develop my character into a moderate fighter, then pump INT while drinking elixirs to finish off my fighting stats.
2) Standard tactics whicn normally win me the game...
I don't know what you consider top tier. My top tier is Silvermist Rangers with 70k invested in their upgrades. Most of your fielded army should be ranged, including your infantry line - strong shields and thrown for them, I use CKO for this (board shield, curved axe, and javelins). Even elite units should melt when charging you. There is always room to get better - some micro management is needed in most battles.
3) Seiges - 1,500 to 1,800 armies beseiging my poor little 400 garrison castles...
Don't defend castles. While the enemy is marshalled against one of your castles, take two of theirs. Chances are they will break siege to attack you, but they'll be too late. Engineering 10+4 is good for this, virtually instant ladder sieges (~3 hours), and minor delay for towers (~6 game hours).
4) Very stupid realm AI, very clever enemy AI...
I don't know much about marshal AI - be the king, be the marshal.
5) When being seiged, the best troops do not spawn first...
Again, with few exceptions, you shouldn't defend fiefs. Unless you happen upon a battle already in progress (they are actively attacking, not waiting), and it's a sure thing. Attack faster and harder than your enemy.