macOS built-in spell checker performs badly - what can I do? macOS Mojave cannot detect that I made a mistake at badd and cannot guess that "approximately" was spelled incorrectly.

How can I improve the built-in spell checker? (i.e. I don't want to use a spell checker inside Microsoft Word or similar third party application that has its own spell checker)

There is a good thread here, but no solution either: 

https://discussions.apple.com/thread/7502895

I also tried Grammarly. That company promises in tons of online ads that its app and browser extension can help you identify “errors that other spelling and grammar checkers just can’t catch.”. 

“...First Do No Harm” would be a better founding principle. IMHO Grammarly is not a good product because it caches many errors that aren't. Take any error-free text and look at what Grammarly does with it. Here, the Universal Declaration of Human Rights:



Next I tried SpellCatcherX who was 100 times better when it worked, but it doesn't really work anymore.

http://www.rainmakerinc.com/products/spellcatcherx/

What can be done to improve this?
There is a small application that runs in the background called popclip which has add-ons to check your spelling. Seems to work almost everywhere. It has many other add-ons to help you work. I've been using pop=clip for years with no problem. The spell checker seems to run everywhere and if you select the misspelled word and double-click it, popclip presents you with an alternative spelling.