I knew of Grayjay but did not know it was open source now. A lot of the app is written in Kotlin which is promising for cross platform

Cross platform

None of the are available for both mobile and desktop

It can be cross platform. On mobile and desktop

True. I did make a feature request to have the Newpipe Extractor in the app

https://github.com/mu-fazil-vk/FluxTube/issues/44

https://github.com/ivpn/desktop-app/issues/290

I made this feature request to IVPN. I doubt IVPN will make it happen but I also did it to get the idea out there. I do think IVPN clients are the best FOSS VPN clients on the market and the idea was to fork IVPN desktop and mobile clients and modify them to bee these universal VPN clients were any VPN provider can integrate these clients into their service. This way a user can subscribe to a few or several VPN providers and access them all in one client, easy to add providers in the client. All a user needs to do is add a URL or IP address in the subscription settings of the VPN client, and login to the VPN account and from there the VPN client will import the VPN servers that VPN providers has and always keep them up to date when the VPN providers adds or remove servers.

Also such an idea will ensure there is a one, secure and fully open source VPN client that works with many VPN providers, and VPN providers do not need to spend time and money developing their own clients for desktop and mobile, and can instead spend time and money on their service and servers. VPN providers can contribute to the universal VPN client if they so wish.