You can't define what is present in flatpak or snap without some sort of reference to the application required. So if you know the name of the application you are looking for you would do the following from a terminal as it searches the flatpak repos and snap repos for said application.
Let's do an example of Inkscape, a great free Vector Drawing Package that I used and persuaded the service I worked for to adopt to create accessible tactile images without having to spend £'00s on bespoke tactile image software.
sudo flatpak install inkscape
The terminal will come back to see if there is a flatpak package and if it does loads the installer and asks if you want to continue with either 'y' or 'n'.
Similarly with snap
sudo snap install inkscape
With regards to App Images you would need to visit the particular software website.
This is how it works:
So for Inkscape App Image you need to visit the website:
Synaptic Package Manager is my preferred alternative as it offers more solutions as I outlined in the Unofficial Manual I produced for Zorin 15 (Page 99-102).
In terms of the initial question, you need to ask yourself when will the LTS of your choice expire? Ubuntu 18.04 on which Zorin 15 was based was to be EOL in April this year, but Hardware element supported beyond that. As Distributions reach end of life web browsing might become more difficult to do if the browser doesn't get updated to the latest version.