Yes even diamond/graphite veins are huge, although obviously diamonds make up the minority. The vein will be like 80-90% graphite.I actually am randomly finding - or rather not finding - diamonds, at least until I have found a vein. That's why this is so frustrating. Also, four chunks apart? Are you telling me even the diamond/graphite veins are of that size? In that case there is something I can do to increase my chances.
As for GT oregen in general, I think it works best for people who like to spend some time early in the game randomly exploring the underground - right from the start, I found some stuff I'll need later rather often - not having to look for Redstone, Antimony, Aluminium or Manganese once I get to the point where I need it is really nice (and it's why I keep going). If you're looking for something specific and rare, however, it tends to get frustrating - I see more of that coming when it comes to Nickel. I like the sense of achievement you get by discovering a new vein. However, I'm the type of player who always looks forward to the time when I spend more time building than mining - and then I end up spending two and a half hours cutting away stone in straight lines. I would have an easier time putting up with it if there was some sort of scanner somewhere up the tech tree that makes this less necessary in future, but I haven't found any. I think I might end up liking GT's tech progression better than RotaryCraft's, but the way RotaryCraft makes certain resources almost a non-issue as you progress up the tech tree definitely suits my preferred playstyle better.
On the fun side: I also spent some time figuring out how to progess to level 3 mining equipment without Thaumcraft in the pack. As far as I could determine, you need to build a blast furnace (can be low-tier) and process chrome and titanium dusts you get randomly from certain level 2 ores, then make the first level 3 pickaxe heads from chrome or titanium. Using Thaumcraft shards is way easier though. Not only are they more common if you're still limited to level 2 mining equipment, but making a pickaxe head from them only needs a file.
There is a scanner. It has crap range, like 20 blocks or less, but the OV scanner will summarize the top 10 or so ore finds within range. Test it in creative, its neato. Its really nice when you're branch mining for ultra-rare veins and you don't wanna miss one by just a couple blocks (passing above it for instance).
For tier3 picks: check this out. I can't promise its 100% current, but one thing of interest is the cobalt: You can smelt that stuff right away without any special equipment. It also lasts a long time, so it makes great tools in general (files, hammers, etc). And as you've seen, thaumcraft shard tools are awesome.