Hello 
There are some other you can try.
Discourse Icon Theme Component
With this theme component you can display icons in posts.
I mean the is maybe a good option.
[wrap=icon id=far-heart][/wrap]
Plus you can add a watched word for this to easy replacement in /admin/customize/watched_words/action/replace
For example:
far-heart word replace to [wrap=icon id=far-heart][/wrap]
Make sure to check in the HTML checkbox.
The second one is probably not what you want because it globally change the 
emoji for everyone.
Same like the first option: Use watched word replace to change the :heart: to ♡.
I would prefer the first option.