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You seem to have a lot of knowledge about something you really know nothing about. One of the factors that makes larger mass boilers comparitively inefficient is the amount of mass involved. If a boiler can heat quickly enough to start delivering heat 90 seconds after a thermostat call why maintain a buffer tank?
The only time a buffer tank is recommended is for a radiant installation, where using a buffer tank as the heat source instead of maintaining temperature in the boiler makes more sense and is more efficient.
Even a properly sized boiler is oversized on all but a few days of the year. In the shoulder seasons, as you have called them, the oversize factor tends to make boilers even less efficient. However, if most of the heat that has been generated is distributed to the heat or hot water when demand is satisfied, the effects of oversizing are nil.
As far as the domestic hot water, PHE's are extremely efficient and very fast. In the small percentage of installations where the water is hard enough to cause fouling, treat the water with a scale preventive or install the PHE as a heating zone and there is no issue. The system is not designed to make instantaneous DHW for the fixtures, it replenishes the hot water being drawn from the storage tank. With a properly sized tank, running out of hot water or having to wait for it to be made just doesn't happen.