You’d be correct - immutable structs basically don’t allocate and can live mostly on the stack. Having an immutable struct and “recreating” it over and over isn’t a problem in julia. You can convince yourself of that by benchmarking some non-trivial function with e.g. BenchmarkTools.
That being said, using a mutable struct and modifying it is fine as well. It really depends on your performance requirements for how you want to use this struct.