Different priorities than me.
I never pillage if I'm engaged in conquest - I want those towns working for me! I won't leave my mounted units alone to die - a kill ratio of 1:1 isn't good enough for me - I want 1:4 at least and in practice probably get much higher - in a war where I have rifles and the AI doesn't, it might be 1:10.
I agree though that seige isn't too slow. However I can't reach a ratio of 50% seige. It leaves my army too vulnerable. A typical renaissance SOD for me might be:
10 rifles
8 trebs/cannon
4 knights
2 pikes or other older units such as crossbows.
8 muskets or maces
Usually after the first major thrust this will get split into two armies which lets me take cities quicker but means that I don't have enough seige to take cities down in less than 3 turns.
At that point an army looks like
5 rifles
4 trebs
2 knights
1 pike
4 muskets
And it might be attacking a city with 12 defenders. Decreasing the number of units I can use to attack and increasing seige means that even if I can deliver maximum collateral defenses and take down the city fast, I don't have enough units to kill all the defenders in a turn. Which means they just get reinforced the next turn and I've changed the problem from dropping city defenses slowly to one of killing slowly.
I'm not sure what the perfect balance is - but I like having a mainly infantry army. That way I don't fear counterattacks or flanking units so much.
I find this hard to relate to my games in this era. You don't have enough seige in your SoD if you only have trebuchets and rifles (unless you use spies to revolt castles) and if you have cannons then the rifles are just icing on the cake and you could have already have won the war with cannons and grenadiers
. You seem to be having your cake and eating it
I find that if I go for rifles as the breakthrough military technology then I have to accept that for 30 turns or so I will not have access to cannons since they take another 10 K beakers of research from a beeline to rifles. And vice versa if I beeline to cannons then it takes 10 K beakers to get rifles so I may as well pick up grenadiers and use those as my top troops and then I put off rifles for a longish time while I attend to economic matters. Cannons are so effective anyway that almost any troops can clear up what is left and macemen and muskets will do fine. Upgrading CR macemen to grenadiers is enough to take the toughest defender in that age when backed up by cannons. With a few cannons to bombard castles you don't need espionage to cause revolts. If cannons are your military breakthrough technology then you need to build a lot of them and only a few support troops, so your stack doesn't have enough seige.
But if you go with rifles and trebuchets then you need 9 siege to reduce castle in 3 turns. Then you need to use some trebuchets next turn to weaken defenders for any troops other than well promoted rifles (e.g. CR2 upgraded from macemen), otherwise the other units in your SoD like knights and muskets and even draft rifles will take casualties against the best defenders. So I would argue for taking at least 11 trebuchets if you want to attack on turn 3 as that gives enough spare trebuchets to soften the defenders, as it takes 25 trebuchet turns to bombard a castle to zero defence.
The only scenario I can imagine where what you are aluding to (i.e. rifles and cannons in the same stack) might occur is if you had researched rifles (say) and then traded with several AIs to rapidly get cannons as well. I don't like trading military techs with AIs as it helps them when I want to crush them. Of course at the end of the renaissance you can have cannons and rifles together and then you'll need them both as your opponents will be fielding either cannons or rifles themselves. But I am most interested in exploiting the military breakthrough of either cannons or rifles.
I don't really understand what you're getting at with the half sized stack. I would not send that against a city with 12 defenders
. You list 4 trebuchets which implies that it would take 7 turns to reduce a castle to zero defence. I might split my SoD in half send a detachement that size you listed against a weak city with say 5 obsolete defenders and only walls or low culture. The 4 trebuchets would then remove the defence in 1.5 turns and a couple could soften the defenders for superior troops to finish off. Against 12 defenders or castle I would send your full stack.
Of course all games are different and even the units are different when promotions are taken into account. I am not saying what you report is wrong just that you would do better to take more seige in your stack as it would speed up conquests usually taking a city 1 turn sooner in the case of rifles and trebuchets (i.e. 3 turns instead of the 4 it should take you).
Edit: reduced estimates of beakers after looking at tech tree. Memory plays tricks on you sometimes
as you might remember from my posts in your thread advocating close packing junk cities into spaces between normal cities. They were my "drafting cities" and I often draft from them several times in the first 10 turns depending on their happiness cap. I know how to raise a drafted army 
but it still takes a lot of effort to get both rifles and cannons if you are at war. There are so many other techs needed to make a war of expansion. I suppose I tend to take the espionage variant and so I will have researched Constitution (for Representation in a SE and Jails) after Nationalism (for Nationhood and Taj Mahal) before doubling back to research Gunpowder and trading / researching for Machinery and Engineering and Theocracy. In the GA from Taj Mahal I switch civics to Nationhood and Theocracy and start drafting muskets while building macemen, pikes, trebuchets and spies. That is the army I use for the first conquests in the middle game. And I don't underestimate the power of drafted muskets as part of a well balanced stack 
