I need a better Venetian stack

[FactionHeir]

While Milan is different from Venice, it is only so slightly, so I'll tell you what I used as Milan..

I basically made up my army of 1-2 generals, depending on whether I can find some troops to occupy another general with or not.
Secondly, I use cav heavy armies, so usually 4-6 knight hospitaller + 4 feudal knights.
Add to that you'd want 2-4 dismounted feudals and some spear infantry such as armored sergeants or merc spearmen.
For ranged I'd then suggest mounted xbows for speed or merc crossbows for stationary. Genoese xbows work as well, but tbh I actually never seen any for hire (maybe after gunpowder age?), which is why I suggested mercs.
Caroccio may give you minimal morale boost but its too few people and I found them to be overwhelmed by rebel peasants already so I wouldn't recommend using those except for a defense.
For artillery, you'd want ballistae to take out enemy artillery and trebs for hitting people and walls. I found the AI using trebs are very very accurate, more than catapults even against troops and a lot more devastating. I suppose 1 treb and 1 ballista should really be enough. For sieges you got rams, towers and ladders anyway to scale those parts you don't want to break the walls of.

[Shahed]

Feudal knights mounted or dismounted ?

[Amon_Zeth]

Feudal knights mounted or dismounted ?

Mounted, otherwise I'd have no cavalry other than my general.

[Wizzie]

Hmm, 6 Crossbowmen seems a bit excessive. I would tone it down to three or four, and add in some Feudal Foot Knights in their place The Venetian Infantry are great but I find the foot knights even better. Also, 7 heavy cavalry is great, but I usually try to include some light cavalry (mounted sergeants; broken lances are heavy, arn't they?) in my stacks for performing lightning-flanking manouvres or chasing down routers.

[Musashi]

I typically use my general, 4 VHI, 4 Italian Spear Militia (Or pike militia), 6 Pavise Crossbowmen, two units of cavalry, and three artillery pieces.

[Jagger]

If I expect strong enemy heavy cavalry, I definitely want some Venetian heavy infantry to back up my infantry line rather than dismounted knights.

[Amon_Zeth]

How effective are pike militia/spear militia, anyway? I've heard some people complain about spear units getting destroyed by cavalry charges.

[Handel]

There is no such thing as "better stack". One stack is better for defence, quite other for attack, third against horse archers, forth for emergency help...

[Musashi]

How effective are pike militia/spear militia, anyway? I've heard some people complain about spear units getting destroyed by cavalry charges.

Pikes and spears survive cav charges better than non-spear units. Italian Spear Militia can hold cavalry much longer than VHI, but VHI will do more damage to the cav unit.