A flow temp of 75°C will only give you a one minute cool down period which is insufficient to prevent cycling because you're looking for a 20°C drop.
And as you've already said, a lower flow temp doesn't heat the house enough, there isn't much more you can do.
Radiators are sized for higher temps, flow temp 65°C, trvs on all but two rads, auto bypass turned down as flow through non-trv rads is sufficient. Cylinder is blue jacketed about 5m from the boiler, so nothing special. Hw is timed and cylinder stat turned down to prevent boiler cycling near...
Well, I have a 415 which works 100% and doesn't micro fire. The important thing is to match the flow rate with the first minute's firing (about 75% of max boiler output) and subsequently d0. However, you're stuffed from the start if your boiler is significantly over sized.
A 28Kw boiler...