As the desktop environment to replace Aqua.
Main web browser with this theme to replace Google Chrome (Google…) and Safari.
Like I do on macOS, I would use Ungoogled Chromium as my second web browser.
Other good alternatives: Midori, Min, Basilisk and uzbl.
This is the BitTorrent client I'm already using on macOS.
qBittorrent also looks pretty good but I've never tried it.
Firewall application that monitors and controls outbound internet traffic to replace Little Snitch.
For video conversion/transcoding. It's already what I'm using on macOS.
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And if you're not into Firefox (waterfox being outdated), try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.