Yeah word replacement is exhausting, and it does feel like the two fluent states for me are extreme relaxation and a very specific amount of localized stress. Lots of... unnatural pauses. Sometimes warmup sounds like "uhhh" that can inject a kind of vowel before a word. Have you tried listening to an AI version of yourself speaking fluently? I have and sent it to some friends – one of their points of feedback was that the cadence isn't like mine. My cadence is built around stuttering avoidance!