Recommendations for an open-source relational database implementation in AWS?

For a few years now, our company has been using a third-party SaaS called Knack, which is a relational database software company. Unfortunately, as of late, the company has become unreliable and problematic. As a result of an influx of bugs, loading problems and other issues, we are considering trying to create our own in-house solution.

While we can take our data with us, we cannot take the "structure" of the web-access for the database with us, so we will have to rebuild it all from scratch. Due to this, we would like to take an approach using an open-sourced software that we control ourselves, so we always own BOTH our data and our structure.

Are there any recommendations for relational databases that also come with web-access (so we could design data views and update forms for our users) that could be effectively implemented into Amazon AWS? I've had a difficult time coming across open-source solutions, and was curious if there are any company devs or admins that have gone down a similar route and have a solution they'd recommend.

Thank you, Michael