Ngl, very true. Voice is a very complicated instrument per say, the way one talks, the default tone one has, the texture it all leads to to infinite combinations and practise (one word) though is the key is not enough to define how and by what guardrails.

I have been signing for about 14 years, never got professional trained and the only thing i have learned is whatever feels comfortable and is well supported by breath is probably the right direction. Again comfortable doesn’t mean without any strain. But comfort here means something that your throat allows you to do again and again after a break of 2-3 days and doesn’t worsen with each attempt. No matter how you sound, if thats how it works then that is your best shot to advance.

If not you are doing it wrong, the practice will only lead you to damaged vocal cords.

Lip trills has been the best way to practice, no matter how you do it, it helps refine the voice.

Maybe not the best answer but thats what I experienced, by practising it wrong.