Synaptic package manager and the GDebi installer are traditional ways of installing applications via the standard repositories. So you know all that and of course using “apt” in the terminal. The drawback is while using an LTS for Debian or Ubunutu, these packages in these repositories are older or not the newest version. More on this later.
Best starting point regarding Snaps is the Snapcraft website. For Flatpaks, look up the Flathub store. Also look up AppImages.
The key positives of these pre-packaged containerized applications is that they have all their dependencies included within a somewhat contained environment and are distribution agnostic. Thus, they will work the same on Debian, Fedora, Solus, or Arch for example. Also, it addresses some of the inherent limitations of an LTS having to wait around two years for a new LibreOffice version. Since all the binaries are included, you can be on Ubuntu 18.04 LTS while using the latest LibreOffice.
The drawback to this approach is that since everything is included, there can be a lot of duplication on dependencies with Flatpaks/Snaps/AppImages. You can have 4 separate apps with only only one dependency needed for all 4 apps but they are repeated 4 times thus using up more space. Also, often they tend to load slower.
Ubuntu is going more to Snaps lately and your experience with Chromium highlights some of the quirks. Also, Snaps belong to Ubuntu and only Ubuntu. Flatpaks and AppImages are not proprietary in that sense.
Snaps and Flatpaks have the advantage if you are on a Debian or Ubuntu long term support release since it could be several years before the next LTS. This allows having the benefits of a stable operating system with the ability to install newer applications. If you are on a rolling-release like Manjaro, KaOS, or Solus, then there is no benefit except when maybe the app you need is not in the regular repository but is is available as a containerized Snap/Flatpak/AppImage.
Hope all this helps you make more effective use of Linux. Take care!