Not trying to breach harm reduction advice or anything, but honestly - all the doses you have mentioned are incredibly low. It actually sounds like you have nearly NO opiate tolerance. So while your tolerance would make it incredibly dangerous to mix benzos and opiates - the doses you've mentioned are therapudict (not recreational) doses. Honestly it would be mind-blowingly rare/unlikely to overdose from
00mg + <20mg of diazapam (valium). Not that I am saying not to be careful; benzos and opiates are absolutely a recipe for disaster and I'm sure people have OD'd on very low doses of incredibly weak drugs (like codeine and/or valium) but it must be incredibly uncommon.
honestly - the drugs you talk about are the weakest examples of benzos and opiates and the doses you refer to are absolute minimum / threshold doses.. Most ODs are more commonly related to stronger benzos (like xanax) and stronger opiates; like heroin, methadone, etc.
So, whilst always practice safety and caution, I definitely wouldn't blow your buzz by being paranoid about it. Those are really, really borderline doses and well within the prescribed ranges that a doctor would put someone on. Opiates + benzos always pose a risk, but if you're going to take the combination, I find it hard to imagine a safer choice of drugs and dose than what you mention.
EDIT: Also, I didn't mention dihydrocodeine - it is also a pretty poor recreational opioid - it offers very little euphoria - although, it is good for pain and it is EXCELLENT to help get through withdrawals of more potent opioids, but it really doesn't give you much of a buzz. Further - 30mg is WELL below a threshold recreational dose..