Interesting list, mixing libre software with closed freemium.
Thanks for putting a light on Oni :)
If you prefer KDE integration:
For mail, there's also Kontact which can does what thunderbird does and more (cal, contacts), in a KDE integrated style.
For screenshots Spectacle is integrated with KDE, it's got timing options. Although flameshot is great to edit on the fly, you can edit with a almost-light editor like Krita.
And if you're not into Firefox (waterfox being outdated), try
https://github.com/Eloston/ungoogled-chromium instead of Google Chromium (which is bloated with spyware).
Great list, but didn't specify what each one is equivalent to on the Mac. For me, finding Typora.io was a revelation since it is the closest to Bear Notes that I've found so far.