You are still allowed to mix in pure ajax (axios) calls to get data for specific sites. I do so in some pages where I just have load some extra data manually to use on the page
But I get around duplication by having all of my code in reusable actions. So if I need to get the same data in multiple controllers, I just call the same action
At first it took some getting used to and I did also feel a bit trapped by how it worked. But before long I learned how to rely on inertia to load 99% of my data without any problems