Is the separate WH on all of the time? How is the boiler involved? Not clear to me. Some boilers are better at domestic hot water than others since they can handle cold starts (cold return water) better than others. On some the shock of huge differentials can damage them. If your boiler can handle the cold starts well, and doesn't have much standby losses, it's a good candidate for an indirect WH tank. If the coil in the boiler makes the domestic hot water by an imersion process, then there will always be a delay unless it maintains the temperature required to produce that hot water. If the boiler is supposed to maintain 160 always, and it is down to 80, then it may be the aquastat.
An indirect tank may be cheaper to run, but adding the zone and buying and installing may take a long payback period. They work well, though.
If you dont' go that route, using the boiler's imersion system to preheat that going to the WH during the heating season would save on the fuel used in the WH. Typically, a boiler is more efficient than a fire-ed WH. An electric one is about as close to 100% as they get, though.