You'll have to go with a tail bag I suspect.
We run two of these: https://www.aliexpress.com/item/1005008933844155.html Not explicitly that link, but that "model" - you'll see a bunch with different logos, some without. I also managed to pick one up for really cheap off Amazon so it was cheaper and got delivered faster than my AliExpress one. It does expand a bit and I was able to squeeze a Shoei Neotec III medium inside (!). Approximate capacity is 11-15L.
I've just ordered this one from AliExpress: https://rhinowalk.com/products/23l-35l-abs-hard-shell-tail-bag-with-expandable Will have to see how it fits on the bikes at home, but 23L will absolutely be able to fit groceries.

It's not awful to consider the anycubic Kobra X, takes a rather unique way of handling multicolour.
I'm personally using a qidi q2 combo, as I wanted multi colour and the ability to print ASA well... And not spending over 1k AUD.