I guess it really depends on your volume. And the amount of help you have/are willing to hire to take care of the fulfillment side. I would think Amazon would be easier, at least for US/Canada. When Amazon does the fulfillment, do they handle any shipping issues to the end customer? Don't forget if you DIY, you will have to manage that as well, which obviously you are already for all other countries you ship to. What (rough) percentage of sales go to US/Canada? That may be your deciding factor as far as increase in workload goes.
I receive a produce monthly (box to treats and toys for my dogs) that is shipped via DHL, I was under the impression that DHL quit the US market, but this uses their SmartPost service which takes the package to a specific USPS facility which then does the final delivery. I've never missed one, it's just infuriating watching the tracking info as it goes from about 2 hours away from me to the north, past my area, to a place about 2 hours sough of me and gets handed over to the USPS, which then brings it back to my local post office. However, they all (Fedex and UPS) also now do the same thing, I rarely have a Fedex or UPS truck in my neighborhood, last mile delivery is almost always USPS. The difference is that both Fedex and UPS have local hubs, which are also close by the local post office, so packages go there, get tendered to the post office, and are delivered to me.
Bottom line, if you feel you can deal with the work involved and not get overly discouraged doing probably the least fun part about designing and selling a product, go for it. I ordered my BM235 before you started selling on Amazon, and the delivery time was quite reasonable considering it came halfway around the world. I was pretty impressed. I take it Australia to the US isn't much of a problem, unlike some countries.