A writing system being phonetic would be impractical, because most languages have tons of little phonetic alterations of individual sounds depending on position in the word/syllable, regional variation, etc.
What you usually want is that the writing system be phonemic, i.e. that there is a 1:1 correspondence between phonemes (meaningfully distinctive sound units) and characters. Unfortunately, languages evolve, so even if your writing systems starts out as more or less phonemic, over time the sounds of the language will drift and inertia will usually keep the writing system not fully in sync with these changes. This is particularly bad in the case of English, where there's never been a proper spelling reform accounting for the corresponding sound changes.
English should just abandon differentiating vowels all together. All dialects of English shwa unemphasized vowels to some extent, and the different dialects largely boil down to how we pronounce various vowels.
What you usually want is that the writing system be phonemic, i.e. that there is a 1:1 correspondence between phonemes (meaningfully distinctive sound units) and characters. Unfortunately, languages evolve, so even if your writing systems starts out as more or less phonemic, over time the sounds of the language will drift and inertia will usually keep the writing system not fully in sync with these changes. This is particularly bad in the case of English, where there's never been a proper spelling reform accounting for the corresponding sound changes.