If you are to use Avalonia, which I highly recommend, it may be best to start with WPF. The Visual Studio Community Edition has really good IDE support for WPF and there is a huge amount of information on how to do things in WPF which thankfully hasn't changed in a very long time. Plenty of old books and blogs that go into all manner of detail. Once you have what you wanted working as you want it is straightforward to port the WPF to Avalonia for cross platform support.
Yes this would necessitate using Windows, an OS which I abhor, but as of now . In time I hope that will change with improvements in both JetBrains Ryder and Avalonia.
Both frameworks have various packages that add OIDC support to them so that shouldn't be an issue.
As for beginner - there's quite a bit of learning curve in terms of writing GUI applications themselves. Avalonia is kind of like WPF* but good(tm) and cross-platform. Uno platform is more like Xamarin and MAUI except in a better shape as of today. From technical perspective, Avalonia focuses on consistent rendering on all platforms with Skia while Uno tries to use native controls instead. I generally would not advise using Avalonia for mobile over Uno. In mobile case you might actually want to also give MAUI a try as, despite harsh criticism it has been receiving, it has improved significantly.
* either platform supports various declarative UI libraries (that are either part of their ecosystem or community-maintained), so you are not married to XAML thankfully.
Performance on mobile devices and UI libraries. Avalonia ecosystem is predominantly focused on desktop, which is not the case for Uno. But I liked the fact that setting up and AOT compiling Avalonia templates was a breeze so use it for side projects. For something more serious that has to target mobile, I would investigate how much MAUI has progressed in terms of quality and if problematic, would just go with Uno.
