@Marvin is it possible to restrict a certain flow to a max concurrency limit of 1 (to prevent it from running if another copy of the flow is still running) without code changes? (ex: do it solely through the UI)

Yes, you can restrict a flow to a maximum concurrency limit of 1 through the Prefect UI, without making code changes. Prefect allows you to set concurrency limits at the work pool, work queue, or deployment level. This can be done via the Prefect server UI or Prefect Cloud interface. Here's a link to the relevant documentation for more details: Prefect Concurrency Limits.