> strength training definitely does not have a smooth progress bar

Not at intermediate or advanced levels, but for beginners on a well designed program like “starting strength” it typically does.

Right. Basically, beginners in any sport are naive to effectively all training stimuli, and so they will respond to a wide variety of inputs.

Once you get adapted it gets harder and harder to find new stimuli that will trigger new adaptation without breaking you down too much. If you're interested in running, Steve Magness and Jonathan Marcus talk about this quite a bit in the On Coaching podcast (https://www.scienceofrunning.com/podcast-2?v=47e5dceea252).