Nah, Harry did a lot of work in Metal (force rings, shield bracelet, bear belt buckle, svartalf Summoning circle, lighting chain). He even laments in SG that he'd not yet had the time or money to rebuild his metalcrafting capabilities, which is why he moved some of his standard enchantments to his staff. But have you ever tried to carry around a 2"x6' staff made of steel? Or even a 1"x18" rod? Not going to be winning any footraces carrying those. Wood is traditional, is far easier to work in general, and makes a far lighter final product. Also, if a wooden blasting rod explodes from being overloaded (like in Changes) its going to do less damage to it's wielder than a metal rod exploding (with or without molten shrapnel).
These are all very special tools, the more versatile foci are wooden. At least those we've seen. Elaine has come closest to a versatile metall focus with her lightning chain. Which Harry promptly copied and taught the Warden trainees about.
I agree with your point about the weight. It's definitely a factor wizards consider when they are preparing their weapons.
Easy fix. Under-barrel blasting rod.
If it were easy and/or practical, we'd have seen it by now. Look, you keep arguing that the older wizards are somehow being stupid about all that modern stuff. Not in those words, maybe, but in essence, because being hidebound and arrogant is a dumb way of being stupid when your life is on the line.
That can't be it imo, because being prepared is what wizards are supposed to be good at. It just doesn't make sense that the more experienced ones would simply ignore the possibilities of firearms. Especially not the Wardens, who have full-time jobs as fighters/police. They see their younger colleagues using them. They put a lot of time and effort into deciding what they carry into a fight and into maintaining their gear. They make choices about what they carry and what they leave behind. Their life depends on those choices. And the life of the people they protect. They literally can't afford to ignore a good idea when it comes to weapons.
btw: Older Wizards do use guns too. Here are just two examples:
Ebenezar has a rifle in his car in Blood Rites and he shot at Kincaid with it. Last sentence in chapter 29: "And then both of them went for their guns." First sentence in Chapter 30: "Kincaid was faster." It goes on with Ebenezar rending Kincaid's weapons useless with magic.
Morgan shot Peabody in the end and his shots were very precise and spot on. Seems to me that he was a good marksman, which suggests training. He just doesn't use guns as his preferred weapon in a fight. As he's dead now, we'll probably never know his reasons for this, but he was considered one of the best fighters the WC had. That's sort of connected I think.
Imo, it comes down to this: Each wizard decides carefully which stuff is worth the effort in both creating and maintaining it and when it comes to the action, carrying the weight around that might slow them down. So we have evidence that some wizards - apparently the younger ones - prefer guns, grenades and such things, while others aren't
seen using them. (They might still have a gun somewhere in their inner pockets). Now imagine you were the one who's going to fight them. Are you really assuming your opponents are just being stupid or are you planning for a fight that might kill you?
Consider this too: There are some comments from younger wizards or fighters (Sanya for example) about how progressive they are being by also using guns and such. There is none from an older wizard (or from Michael, who used to be in the army so he must have had basic training with weapons) about how stupid those kids are to rely on them. Despite the fact that most of them apparently have decided at some point in their career that they'd rather bother with something else. The conclusion I'm drawing is that they have their reasons and they respect the choices of the kids because they might know from experience that their alternatives might not yet be available or really better for the kids.