What did you replace Things and iA Writer with?
All of these were replaced with org-mode and Emacs (and likely some other Linux features for Alfred). Doom Emacs is a good framework to explore Emacs as it supports both vim and Emacs bindings (there’s also cua-mode).
With org-mode I have dozens of files for managing my projects and tasks. And I can mix notes with tasks. This is a feature that makes it exceptional. When I was using Things3 I liked the tiny section beneath the task description where I could type some notes about the task. With orgmode I can write a full outline beneath the task description.
Emacs is extremely flexible as it can be extended with packages written in Lisp. There are packages that allow me to navigate to any outline node in my ~ 200 org files within a second (ivy + counsel + ripgrep-integration, also org-ql which allows me to query my org files).
Also, I write large amounts of text with org-mode (it’s an excellent outliner too; been previously using OmniOutline) and convert to markdown or any other format via pandoc.
Emacs, once mastered is an application framework that allows me automate any workflow for which I previously used AppleScript, DEVONthink, Python, Bash.