Your points are all valid but unfortunately doesn't change the reality of the game as a whole.
No, I don't ignore garrisons. I learned to purchase them to a minimal level for
every border territory, no matter if there is enemies at the other side or not. Sometimes even inside territories too. If I remember well (been over a year since I played last), the race I played had a caster at garrison level 20 which drastically altered the battle on my own advantage.
Even then, computers can manage rather easily to get enough forces somewhere to destroy garrisons. Castles are nice and all but the ressource system in D3 make it so it is extremely counter-efficient to have many castles near each other (because every castle take a certain % of taxes from neighboring territories and if your castles are efficient enough they simply "steal" efficiency from each other in a given territory or two) and they are also extremely costly. Sure, they offer nice defense but then again if you can't defend them the enemy take control over them and you're in trouble wasting good time and resources (and supplies, so possibly men too!) to get it back. I only built them in key area for cash or in chokepoints, which were heavily defended anyways.
Maybe you got the impression that I didn't do well in D3, but after a while I got fed up and went for walkthrus and strategy guides over the internet. And if you want a powerful nation then I strongly feel like you absolutely have no choice than to read all those walkthrus and strategy guides not only to build your race (God) right but also to know what the enemy can throw at you. With the number of races available, in 3 different kind of ages no less (some races even having completely different play styles in every age) it is a daunting task to say the least.
My best race was a first age blood magic race which relied on two of their special units to rule the battlefield. The first unit was a footman which turn into a werewolf but which is also linked to your God (I don't remember the term, holy unit?). The second was a caster which could transfer all those God goodies on those units with a mass bless at the very first turn of battle. With this setup I was able to own about every single battle given enough men on my side as they were already decent by themselves but with the bonuses they regenerate (regen wounds sometimes too!) and with luck and missile protection and whatnot... They were night unstoppable for the unprepared.
The only real fun I ever had with D3 was with a Middle Earth huge custom map, with the same race described above. I was pretty much rolling over every single nation but the only real fun was more about conquering Middle Earth than anything else.
I really wish I could say that I like D3. The heart of the game is extremely good and I heard it is actually based on a D&D world the designers created and played (thus the strange "hero" special units you encounter once in a while in neutral territories and which surprise you by being alone but leveling your whole army...) and the items and artifacts you can build reflect that as well. But for me, the whole mechanics of the game simply can't mesh well. The game lose points here, there, there, there and in the end pretty much in every single aspect and... There's that reason why I'd rate it with a 3 on 10...