For phonemic spelling systems, it wouldn't even be hard. Start with the baseline of aeiou for the basic vowel triangle. Then let's say that umlaut changes the vowel's "default" backness without changing roundness, ogonek lowers the vowel, tilde makes it nasal, and breve makes it a glide. This gives us aąäãăeęëẽĕiįïĩĭoǫöõŏuųüũŭ, plus all combinations of these diacritics when needed (which should be very rare). Add doubling to indicate vowel length, and I can't think of any European language that cannot be consistently expressed this way; American English should only need aąäeëiįouų.

There's a lot of sound variation in regional dialects of Spanish, from country to country and even regions within a country. People pronounce vowels and consonants differently, and even the "accent" varies a lot (some people can seem as if they were "singing" instead of speaking, to speakers of other countries/regions).