Firstly, congratulations on the big birthday. A Canadian Dingwall is a great idea as a present to yourself. And in my opinion, you should definitely notice the difference between an ABZ and a Combustion - at least, I did when I first stepped into the Dingwall world back in 2010.
I got an ABII from Bass Direct recently, via one of their fast-tracked build slots that Admiralchew mentioned. I would recommend this approach if you know what you want as this is by far the fastest way of getting a custom shop bass from Dingwall.
However, I would emphasise the issue with them being busy. I tried several times to get information direct from Dingwall, but never heard anything back. Mark and Marcus from Bass Direct were able to relay information back and forth and get my questions answered, but ultimately what was missing was the opportunity to discuss options in detail. I had an idea what I wanted, but not exactly, so I really could have done with speaking to someone from Dingwall directly. At the time, the price list and range of options wasn't available on the Bass Direct website (as it is now), so this would definitely have helped at the time. I got a spec that I'm happy with, but would maybe have benefitted from being able to explore all the options better - for example it was really hard to know what difference the pre-finish treatments would make to the end result.
The other issue was that there was a minor problem with the bass in that the nut groove for the E-string was not quite filed at the right angle and there was some buzz/rattle on the open string. I realised fairly quickly that it was the nut but it took a very long time to convince Mark from Bass Direct that this was the issue. In the end, I got it sorted at the Gallery (I'm in London, so taking it back to Bass Direct wasn't that straight-forward) and Bass Direct reimbursed me for it, but there was a lot of back and forth. I think they were trying to be helpful, but the communication could have been clearer initially and a lot of time might have been saved.
So overall, if you're keen for a Canadian Dingwall, Bass Direct is probably your best bet and I can overall recommend it. I think both Bass Direct and Dingwall have been aware that the huge increase in demand has been difficult to manage and they are actively working to improve their processes. But be aware that you might have to be a bit pushy to get the information you want, or in the event that there are any issues.