I don't use roaming on my Australian domestic mobile, preferring to either purchase suitable SIMs before I leave or purchase one in country. For the camino, where WiFi is almost ubiquitously available at bars, albergues, etc, I don't think you need a large amount of data, so I choose a low to mid range option for that, and not a high end data plan.
On banking, I use a combination of three credit card vendors that I already use at home, and a debit based travel cash card that can be loaded with several currencies. The cash card is linked to my Qantas Frequent Flyer card, which reduces the number of physical cards that I need to carry, albeit that isn't a big issue most of the time. There were only one or two times that I had any issues with it in over three months in Portugal, Spain and the UK this year.
I still carry cash when on the camino. This year, I think it was only around 200 euros, and I would top it up when it got below 100 euros. A lot depends on the route, and for routes that might be through areas with fewer major towns, I would expect to increase those amounts.