There is no point measuring things with a refractometer if you can’t correlate it with what your tastebuds say (just like ratios, weighing with scales & timing).
My Clever brews of up to 30-40 min are no stronger than drip brews of much less time. So I am totally sure that concentration is not the issue. When you extract on the low side of normal you can get flat, bitter flavours…at weaker strengths, the bitterness and chewiness fools you into thinking they are “strong” but they are not. Letting them steep longer can make them brighter, lighter and more clarity on the palate.
Not everyone wants to steep long, so if you are steeping short and get overpowering brews, I’d say grind coarser, maybe steep shorter, you can even updose, to avoid that bland, flat, bitter, generic, middle ground. I do both methods and honestly, I don’t have a preference, but the no man’s land in the middle is often truly awful.
There is nothing “standard” about a 3 min declining temp immersion brew, it is very much non standard (always under-extracts), but can be great if very under-extracted, rather than just a bit under-extracted.