Sorry am a bit of a noob, might need a little more handholding here. Which docs should I follow? Do you mean follow this youtube tutorial? Break up the DATABASE_URL and add host, port, username etc separately?
[Update:]
OK I researched a bit and tried this in terminal:
heroku config:set NODE_ENV=production
And then I pushed an update to heroku to reset things:
git push heroku master
And then I added a post in the admin UI. Will update again in an hour after the dyno restarts to see if the data persists.
[Update 2:]
YES!! It works! OMG after 4 days of debugging. Thanks for the help!
A bit of feedback: I read through a lot of questions in Stack Overflow having issues with Heroku deployment due to using SQlite via --quickstart. many solutions seem to revolve around not using --quickstart, going with postgres db from the start, and even not using strapi create project but use npm instead. Will be good to address these mix of solutions because it was super confusing as to what the problem was (and highlight this solution instead).