I would consider just passing along the data directly to the functions that need access to it, rather than storing in a global state. If passing each piece of data along as separate parameters is a bit much, you can always create struct Context { ... }
which keeps tracks of whatever you need and pass that around.
Nothing wrong with using OnceCell
as @[email protected] suggested, but I’ve found that passing it as an argument feels a bit better.
I’ve done something similar to what you want to do. To avoid the issue with each bank having their own interpretation of PSD2 I ended up using an aggregation service. The few banks I looked at also had horrible APIs, which I didn’t really want to work with.
I picked GoCardless for aggregration since it’s free for simple transaction reading, but there are plenty of options available.