A number of different countries around the world have been "mixing and matching" vaccines for some time now. There's no evidence of any safety risks that has emerged yet.
All the studies done so far, indicate that mixing a viral vector vaccine (like AZ) with an mRNA vaccine produces a better overall immune response than just sticking to the one kind. No one vaccine can stimulate all the various parts of the immune system equally, so by using vaccines that stimulate the different parts of the immune system slightly differently, you can get a better all-round effect.
This is explained in the article below, with one part of it pointing out that:
https://www.nature.com/articles/d41586-021-01805-2
On the other hand, there is some evidence that with the mRNA vaccines, the slight risks of myocarditis and pericarditis increase with each subsequent shot, so some scientists are actually saying it's safer to get a viral vector vaccine for your third dose, rather than another mRNA jab.
As stated in the link below:
How the risk of side effects could change with Covid-19 vaccine boosters