tldr, for those who skip directly to the comments:

> Commitfest is a month-long activity that Postgres contributors dedicate to looking at the patches, reviewing them, improving, and committing to the master branch. There are 5 commitfests each year.

Commitfest #40 is starting in November - you can see the current set of patches here: https://commitfest.postgresql.org/40/

What is the best way to find beginner friendly issues/start contributing to the core?

There ins't a issues tab in the main Postgres repo: https://github.com/postgres/postgres

It's not easy to have a list of what is important to be done as there're so many different things.

Generally it's good 1. To find already proposed patch of a moderate complexity and review it 2. To find a reported bug and propose a fix 3. To improve README's in the area you know how they work

what they're doing is clearly working for them, but have maintainers shown any interest in transitioning the contribution workflow to something more common nowadays like Issues/PRs to make contributing accessible to a wider audience?

It is possible if core team decides they are better with something else. No doubt, there is some conservatism in the community.