I want to run, and then figure out how to get those applications into TrueNas - unless they somehow get those applications into the predefined repos that are installed with the platform.
Here is how you would add extra repos in SCALE -> https://truecharts.org/manual/adding-truecharts/
How do you develop / test a new chart? Do I have to have a registered repo (in whatever structure that is) and then push my helm chart data into that repo in order to get TrueNas to be able to try and deploy it?
Fork a repo like truecharts, add your helm pack in your repo. And follow the mentioned guide to add your repo to SCALE for test/develop
When adding docker containers manually, it would be really helpful if I could import a .env file from my local disk for configuring the container (or even load a docker-compose file and then let me select which container Id like to configure) then have TrueNas ask for the .env file referenced, and configure all the ports and directory mounts.
You can always make a suggestion in https://jira.ixsystems.com/ , but i doubt anything related to docker / docker-compose will be supported (at least for now) While it might work, is not supported and cannot guarantee data safety..
From a repo (truecharts (https://github.com/truecharts/apps) or official(https://github.com/truenas/charts)) find out the structure and a general idea and adjust your app. But i wouldn't start with an app that needs a DB.but I have no clue how to get this kompose output ( a ton of separate yaml files ) into TrueNas Scale
Start simple first.