com.spotify.Client VS ice

There is nothing "unsafe" about flatpak. In fact for every package on flathub you can see exactly what the "unofficial" packagers do in the corresponding github repository. For example here is the repository for Spotify, if you look at the json manifest file you can see they are repackaging the "official" Snap with the necessary libraries and scripts to support it running as a flatpak. Everything there is open source and able to be examined.

If you click on the "Manifest" link for the package on flathub, it will take you to where the code that packages the app resides. There you can review exactly what the packager does in the json manifest. Here's the one for the Spotify client, for example, which shows where it pulls in dependencies and where it downloads the proprietary code from (in this case they base it on the snap) and what commands it executes while packaging the flatpak.

Try installing it via Flatpak

Flatpaks builds are made to be reproducible. Everything is done openly and you can check how the apps like for example Spotify where made.

Why not just use the flatpak?

Judging by the manifest, the Spotify Flatpak is built from the Spotify Snap.

There is also ICE.

Peppermint Ice (GitHub): Supports using Chrome, Chromium, Firefox, and Vivaldi as its browser.

Linux Mint has webapp-manager which works similarly to ice; and ice itself has a Github page, but I'm not sure how well they work outside of their respective distributions.

I don't know what you've already tried but for Kubuntu you could try the Peppermint OS Ice SSB There's a PPA

ICE is a good way of doing basically this, including creating the profile and giving it a “hide everything” userChrome.css, when you use Firefox: https://github.com/peppermintos/ice

Depending on which distribution you're running, you can try ICE - SSB Manager From what I remember it's available for the Debian based systems and I think Arch has it in the Aur.

"Should theoretically be installable/useable on any Ubuntu based distro (or any other Linux with some minor tweaking) .. ICE has been available on github for some time now:https://github.com/peppermintos/ice

If you're on Linux, check whether or not you can install https://github.com/peppermintos/ice

The Linux Mint Web App Manager is based on Peppermint OS ICE SSB Manager. There's a PPA