https://youtu.be/M6Z6nCqSMEA?t=17That looks like a proper zoom zoom thinking flooding the boot with ad blue.
So it looks like that they filled the traditional space of a spare wheel with the after thought ad blue tank and there is no free space even for a space saver wheel.
I do not think the road assistance can refuse to go to a car with no spare wheel as most of them do not have it nowadays. But maybe there is a business logic behind that, as they cannot replace the flat tyre wheel with the spare wheel, there is no point in them driving to the car, removing the wheel, driving it to the tyre shop and back. So they may just send the mobile tyre vans.
The waiting time is a thing and when you travel a long distance over many countries, it makes sense to invest in the spare wheel.
Buy a second full size wheel, used or new, but the same as the ones the car uses. Then fit a new tyre to it and keep rotating it every time you fit new tyres.
It seems that it will need to be secured in the boot and you can use some plastic fake leather wrap like the 4x4 type of car.
https://www.ebay.co.uk/itm/One-17-Mazda-6-SE-Nav-Alloy-Wheel-Rim-10-Spoke-A/183264925600?hash=item2aab70d7a0:g:WTQAAOSwVK9bGoFNI say buy the same wheel to make sure that the nuts suit the wheel. If the nuts do not suit the wheel, they can come off after driving. So while they should be cheaper wheels, I would be careful what to fit. As you do not buy them every day, if a wheel the same as the ones you have is around £100, go for it. Any wheel is sold for around £50, and so pay the extra and have peace of mind.