Throughout Stargate whenever the Goa'uld launch an attack on the SGC it tends to come as somewhat 'clumsy' physical attacks. Surely a race as advanced as the Goa'uld would have developed far more sophisticated means of electronic warfare to penetrate SGC systems in order to lay the ground for these attacks. I am thinking of infiltrating SGC networks to open the iris for an assault, mimic the GDO code of off world teams.
In the same vein, why not simply launch chemical warfare attacks remotely
Rather than sending Tealcs son, for instance, with the substance in his tooth, a ploy based on him having to then reach the surface.
Many of their efforts do not seem to be in fitting with the advanced and intelligent species they are labelled as.
Whilst combined assaults would be hard to coordinate amongst fragmented System Lord forces, attacks by lone operators would not face that problem. Especially amongst those not engaged in large scale faction warfare with opposing System Lords. There are minor Goauld who were not engaged in large scale warfare and who had access to the necessary technology, were scientists in their own right etc, who would have an interest in earth as a source of labour, research outpost etc.
A prime example I think is Anubis, who despite his enviable levels of technology, far surpassing any other Goauld, launches an old school fleet assault on earth, it is reasonable to assume he had no need to act as such and could easily have devised more effective method to conquer the Tauri.
Even other Goauld who considered the Tauri an annoyance could have launched highly effective chemical/biological attacks using only a cloaked transport for instance. Granted this may have offended their pride given the arrogance of the average Goauld. Remote attacks would not be rendered useless by the iris as radio signals still pass through and could be used to attack the installations electronic infrastructure. I'm willing to concede that some of this must be due to the need to keep the plot going.
Even when the destruction of earth began to become a strategic necessity, nothing was ever really done except fly big ships and shoot.
The only caveat that springs to mind was when Anubis created or modified an asteroid to contain Naquadah which was then steered towards earth in the hope that it would either collide or that the Tauri would attempt to destroy it with a Naquadah weapon and trigger a monumentally larger explosion.