appcenter-web VS jekyll-auth

There's also a lot of apps written in Vala for elementary OS's AppCenter with screenshots & links to GitHub: https://appcenter.elementary.io/ (not all, but most). I published two apps there and using Vala has been a great experience for creating Gtk/Linux desktop apps.

COVID hit like immediately afterwards so the sprint we had planned didn’t happen, but we did deliver on making AppCenter Flatpak-based so you can now install AppCenter apps on any distro just like you would any other Flatpak remote. You can browse and download apps at https://appcenter.elementary.io

So, something along the lines of the elementary AppCenter?

I'm not sure why you said it is difficult to distribute Flatpaks outside of Flathub. On the repo host side elementary has their own Flatpak repo complete with an app payment system (before even Flathub has one). On the user side I can simply download a .flatpakref from appcenter.elementary.io and double click it, which will open Software with the app's metadata, and clicking Install will add the repo and install the app.

Elementary OS has a pay what you want store. That might be a good route to go down.

Yes, elementary is Ubuntu based, however it stands very far from it. It doesnt use snaps at all and instead uses flatpak. You can also just use apt if you really don't like Flatpak either. The "main" way to install software is through the AppCenter app, which uses flatpaks. They have their own software repository (appcenter) where apps are "curated" (aka all follow the elementary HIG and use the latest SDK version). But you can also install "non-curated" apps (aka from Flathub) directly from the AppCenter.

You probably know that if you use desktop mode, you can use Discover to install a bunch of applications via Flatpak from a repository called Flathub. But, you can add an additional repository from another distro, Elementary OS, to get access to even more apps. They will show up in Discover, just like all others. To do this, simply execute:

elementary os appcenter https://appcenter.elementary.io/

AppCenter, which is focused on open source GTK apps and accepts payments on elementary OS

Elementary AppCenter - setup

Though if you want actual authentication, you’ll need a custom backend or use a service like Netlify (as you mentioned), GitHub Pages (looks like Jekyll Auth is end-of-life, but you can still use private deployments with a paid account), etc.