I don't mean IPA symbols (which I can't read) but rather characters from a normal alphabet being used to phoneticise a word, e.g. excerpt is pronounced "[EK] + [SURPT]". What would this be called? Letter-based phoneticisation?
I believe you’re thinking of phonetically, or spelling it how you hear it.
Phonetic spelling
https://study.com/learn/lesson/phonetic-spelling-examples.html
Do you want to learn the IPA? It's not very difficult.
Here's a simple introdtion vid: https://m.youtube.com/watch?v=OJlF2eiGsP4
If you’re specifically looking for a term that differentiates what you’re describing from IPA, see pronunciation respelling.
I thought both the sound and the letters were referred to as phonetics. Now I'm interested in what other answers are.
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If a language has a better spelling system but doesn't use it, it is a shitty language (English).
I kind of agree, but there is an issue with this.
Even within England, the difference in pronunciation from region to region means that there can be no 'true' pronunciation. Add to that the weird a/o vowel, the t/d thing and odd emphasis on 'r's at the ends of words that many Americans use, or the wholesale reassignment of vowel sounds that New Zealand uses, and you'll find that any 'proper' way to spell things is not how the majority of the world says it.
The biggest issue is that English is an exceptionally fluid spoken language, but began solidifying as a written language hundreds of years ago.
Even if we did a do-over of the spellings in written English, it would fall out of sync with the spoken language very quickly.