A single world language should be designed and promoted. Previous attempts have been too Eurocentric to take advantage of all useful grammatical features that are available.

There are so many constructed languages already that you do not need to design anything, if you have some criteria, just pick one that suits you, brush it up and maybe replace the vocabulary. And then goes the minor issue of promoting it and gaining speakers :-)

English is rather badly suited for an international auxiliary language, as the things go. But still better than French or Chinese, all things considered. Spelling is OK, it's the pronunciation that sucks :-) And its weird syntax and internally inconsistent vocabulary.

Spelling reform (or a completely new, purely phonetic alphabet) is urgent.

You really, really do not want phonetic writing system - phonemic is what you should go after, but morphophonemic has certain advantages too, especially for non-native speakers.

I would advocate adapting Hangul for that purpose.

If you match your phonology to 15th century Korean, sure. Not so much otherwise without a substantial reform and introduction of new letters. Anyway, Hangul is more suited for a morphematic writing system, not phone[mt]ic one.