A simple solution and recommedned approach can be - 

If possible you can club all those common user defined functions in a structured python package / whl file.

Now once this whl file is created you can simply upload it to your catalog volume and the from there
you can access it either through pip install it at the top of the dlt pipeline and can import whatever functions you need.
or you can install the library at the cluster level though not sure that when using dlt serverless how to give library to the serverless compute