Aren't there symbols missing for the mid central vowels which are common in English? I.e. https://en.wikipedia.org/wiki/Mid_central_vowel.

How do you write e.g. "learn" or "bird"?

It seems to be needed in the first "This is a sentence..." example as well. Right now it uses U for the lone "a" and I for the second syllable in "sentEnce". But U represents /ʌ/ (as in but) and I represents /ɪ/ (as in bit), but a symbol for /ə/ would be a better fit for these two, right?