Linux

I mainly use Kwrite and Kate

Kate Snippets

Check out Espanso, if you're ok with something that is system-wide and not a text-editor:

https://espanso.org/

Vim macros are quite easy to use, if you already know how to vi.

If I understand you correctly, this is trivial in emacs:

(defun insert-text ()
  (interactive)
  (insert "your text here"))

(global-set-key your-keybind-here #'insert-text)

You could make it a format string if it relies on data specific to some file or parameter. You could also make the keybind local to certain modes/files rather than a global keybind if you don't want to pollute your keybind space.

I use Autokey because I prefer a global snippets engine. Then it works in text editors, browsers, email clients, etc.

Pulsar (i.e. active fork of Atom) has a pretty comprehensive snippets package that comes bundled with the editor. Can be configured with some fairly simple cson, for example with Markdown:

'.source.gfm':
  'Hello Lemmy':
    'prefix': 'helem'
    'body': 'Hello Lemmy!'

You type helem then press tab and it will expand to Hello Lemmy! when using the Markdown grammar (source.gfm).
It can handle custom tab stops too so you can make a longer preformatted sentence with gaps to insert words which you can just tab through (the $1, $2, $3).

'.source.gfm':
  'My custom snippet':
    'prefix': 'mcs'
    'body': 'My snippet stops here $1 and then here $2 and then continues $3'

You can even do multi-line snippets. For anyone wanting to try it out the docs are here

Visual Studio Code has this feature. You can define user snippets using a JSON format and create any alias you like for each. Then, when editing a document you just start typing the alias and press tab when it suggests the snippet (no mousing around required).

You can also create anchor points for variable content that you then tab through and fill out after inserting a snippet. If you find yourself using the same template for your writing this might be a helpful feature.

I’m bad at describing things, here’s some documentation: https://code.visualstudio.com/docs/editor/userdefinedsnippets.

This is all out of the box with no plugins installed. Also, I’ll point out that although the docs are coding related, you can make these work for any type of writing.

Autokey?

I've used Autokey with great success in the past. Easy to set up, just works.

CopyQ

Kate snippets. If you download them you think "huh do I need to know XML or what?"

But just install the extension and everything has a nice GUI