My method works fantastic for lighter roasted coffee which is roasted for pourover etc... but I just cant seem to make a pleasant tasting cup from medium roasted coffee that is marketed as for espresso. All coffee is within 2 weeks of roast and has been kept in sealed containers.

Method:

1. Grind (on breville dose control pro). The grind size is approximately drip coffee size
2. Kruve between 1100 and 500
3. Preheat 400ml plunger
4. Brew 20g coffee grounds with 240ml water at 90c starting temp for 4 mins.

For lighter roast I get all of the origin flavours, no sourness, no bitterness, really getting all that this grinder is capable of delivering in a great tasting cup. Considering how modest the grinder is, the cup probably tastes better than it should! However, for medium roast coffee (sensory lab steadfast) which for me tastes fantastic as espresso, all I get using this plunger recipe is over extracted crap. Tried dropping the brew temp down to 80c and tasted even worse.

I want to keep the grind he same because it works so well with so many lighter roasted coffees, and also the grinder becomes noticeably less accurate at more coarse grinds.

So which combination of time and temp would you recommend for my medium roast, for a grind size that tastes great at 90c for 4 mins on a lighter roast?

Grind coarser and consider giving the French Pull method a try. Other than that, I'd only change one variable at a time. The pull method will give you a cleaner cup. And medium roast coffee is more easily soluble, so you should compensate with a coarser grind. With any brew method, I always grind as coarse as I can get away with.