Easy spelling!
There are 26 letters in the English alphabet, but 44 phonemes. I'd start there. Expand the alphabet to 44 letters; one letter per sound and double le(tt)ers are not necessary. Thus no ambiguity on how to spell a word; you spell it like it sounds. It would be like a metric system for speaking/spelling...in that it makes sense. So "two" becomes "tu" or maybe just "2" (wi yuz tu karakters wen won wil du?), "too" becomes "also", and "to" becomes anything...maybe "tob".
You nicely illustrate how incredibly hard this is to do correctly.
The word "to" would have to become "tu" (certainly not "tob" as it would sound more like "tahb" - the "ah" sound as in the "won" you use for "one") to keep in line with your previous examples, as it's pronounced exactly the same as "too" and "two". Furthermore you shouldn't use "also" without pointing out that the "o" in that word has to become a new letter, as you have used the letter "o" already in "won" (as phonetic spelling of the number "one").
This is also ignoring the constant spelling updates you'd have to perform to keep track with changes in pronunciation of different words over time (which, in part, is why we have so many spellings in English that do not fully match current pronunciation), or regional differences in pronunciation of various words: which version of English would be the standard? You won't even be able to say "British" or "American" as neither has a standard pronunciation but comes with huge regional differences. Those spelling updates will seriously mess up reading as a large part of reading is done by recognising the word as a whole, rather than looking at and parsing individual letters.
On the other hand, some languages like Latvian have settled down on their spelling only quite recently, and it's possible for a non-speaker like me to read out Latvian text and have native speakers understand what I say, while I have no idea of the meaning of the words, just reproducing the sounds. The same supposedly works in Hungarian, and probably some more languages.