LOL understand man. I'm kinda the same way, like to struggle with something no matter now bad it gets to a conclusion of some sorts.
Keep in mind the Dir Series and DGL series of routers are fairly low end routers and mainly meant for avg home Internet use. Specially the low numbered 600 series. Given that, I do agree, the routers should work well out of the box and not exhibit this kind of problem. Now given that, I presume you have entered the IPv6 information in correctly and you are using I believe, long form of the address? I saw a post not long ago that using the short form of the IPv6 address was not allowed while inputing them into the router.
Given that this router is a low budget router, what are your router needs for your network? Is this a good solution for you? Even if it worked? Is this the best router for your needs? There are other DIR routers that work well and can handle doing more. I do enjoy the DIR-825 I just got one from a guy on the boards a few months back. He couldn't get it to work well for him so I bought it. Rev B and IPv6 ready. I don't have support for IPv6 yet from the ISP so it's disabled and haven't had a chance to play with it yet. That and it's dual simul band WiFi. I wanted to try a DIR series router from my DGl-4500 which has been the greatest router I've ever bought. Must say the 825 is the same now. Just more features then the 4500 which isn't IPv6 ready and won't be.

So keep this in mind.
Now If the OEM FW doesn't work well or is defective. I would maybe contact maybe with one of the moderators on here before calling. Give them the details and ask them if there could be an issue here. I would try this maybe after you do the WRT as a test. You can revert back to OEM FW if WRT doesn't work out for you. Not sure calling CS at Dlink would result in anything however it's worth on try I guess.
Over all, you might think about, is the router really worth it? Does it suite my needs for a router? All the trouble and hassle of getting information from and to Dlink. Might be worth just finding a different solution.
Keep us posted.