You’re right for Pinta. Can’t be 100% sure about Paint.NET to be fully open source but as it based on .NET it might be…
Well, GIMP is great but uneasy, not sure but Paint.NET on Windows is simpler but very user friendly and may be sufficient for your needs
Same, Session is great because it focus on privacy. You can contact people only by knowing their public ID but you don’t know their private ID which is used by the user to decrypt the messages. As there is no central server, nothing is kept outside of your local Session instance. Pretty decent to me.
Keyboard is absolutely a thing to be careful with. On Android mobile, use OpenBoard for example. On computer, if you use Linux, also install kloak [https://github.com/Whonix/kloak], a tool that slightly delays randomly your keystrokes to help you hide your typing pattern that can definitively identify you
Right, but still quite affordable compared to similar solution with Proton IMHO