How to set default decimal precision?

You could add it to the start/2 function of your Application module. For example:

defmodule MyApp.Applications do
  def start(_type, args) do
    D.Context.set(%D.Context{D.Context.get() | precision: 48})

    children = [
      ...
    ]

    supervisor = Supervisor.start_link(children, args)
  end
end

Decimal.Context.get/set are a trivial wrapper over an entry in the process dictionary:
https://github.com/ericmj/decimal/blob/68633fc6651456b80cfee349ef96be0000449468/lib/decimal/context.ex#L101-L115

so AFAIK there’s not a “global” place to change them.

Unfortunately this doesn’t seem to work. @al2o3cr might be right and there might not be a global place to set it

There is no global place to set it, it needs to be set in each process where you want it to be in affect. So if you’re talking about a Phoenix app (which you are I see from above), then you’ll need to set it in each request (as a plug would make sense). And then in each other process like liveview processes.

The flipside of setting the context being really cheap: it’s fine to do it frequently, close to the code that depends on specific settings.

Setting it close to the calculation also avoids gotchas where code runs in another process unexpectedly; for instance, Cachex.fetch runs the supplied fallback function in an internal process, NOT the caller’s process.