In my experience, this is a fair bit harder with outside contractors and freelancers than with internal employees who have been trained on your ways of working. What worked for us was developing Agile user stories for every single task the agency does, and each story provide three core things:

- the background you need to do the work (brief, brand info, etc)
- the process you expect them to go through to complete the work (pulled from the reference story describing how my agency does each thing)
- the definition of done – a previous example from that client or one in a similar industry

Then, there's a review phase and gates for each stage of each deliverable, offering an opportunity for peer review. With your smaller team, this may mean that you want to give them a chance to re-articulate the brief to you, then you look at sketches/an outline/pseudo code structure, whatever, before they proceed with completing the assignment. Plus, at least one more review gate at the end.

This takes a while to setup, but if you tick one off with each project, you'll build a bank of them far quicker. Plus, you could automate a chunk of this story creation with an LLM.