I think the main problem for sieging is you can't force people to change their unit type.
Sure you can ask the whole realm to 'get more infantry!' but if people don't give a damn or they have been with their units for so long their captains have over +10 bonuses, who want to switch over anyway? You really can't do much about unit compositions. Have to work with what you have. There is of course a dirty way to force people to recruit a certain type of unit like getting rid of all archer RCs and etc but it is just a dumb idea which costs a lot.
Sirion has 5 cities. 2 SEs every day will get you 10 a day so to get 100 you will have to wait 10 days. Do you still have to travel to other cities to pick them up or can you pick them up in your own capital? Also, this isn't just about Sirion. How do you expect a single city realm to besiege a city with lv5 wall? You will have to sit in your city for 50 days. Or bring 5 single city states if you want to sack a city in 5 days. I don't know how many we will gain/lose from the limit change per continent but I doubt we will get enough to sack a city like even 2 years ago. Nowadays it is just flat out 'don't you dare attack a city'. Even if you win, you get tossed out by peasants.
I think maybe instead of making people pick up multiple SEs, why not just make people pick up 1 SE per person. So you don't have to make a few people to carry 10 each? Instead maybe people should be only able to pick up 1 per person but more you gather for a battle, the more synergy you get out of?
Anyway, the siege problem is not just one or two things causing it. It is a complex problem affected by multiple changes over the years.
Oligarch is honestly a special case though. Most cities don't have that much CS. I have been in multiple battles where the attacker brought over 100 siege engines. I was a military attacking leader in one or two of them.
You really do need to bring close to that 1SE per 10 men to fully neutralize the lvl 5 walls.
Finally, on the topic of unit type that really only means one thing. Sirion's nobles don't actually care enough about defeating Oligarch. If they did, then people would all drop their archer and cavalry units for infantry.
If they choose not to do so, then the leaders of Sirion haven't done a good enough job of convincing them of the need. If they can't do that, then the attacks should fail regardless. The whole concept of the "they don't give a damn" is your root problem.
And again, Oligarch is a special case. It's strong enough to hold you off, which the Duke apparently planned for. He won. That's how it works sometimes.
I agree something should be done with tweaking some balance, but there are so many things you could do to improve your advantage, it would make sense to start there first.
