Standby losses on electric tanks are fairly low, and can be made almost arbitrarily low with more insulation, so plumbing them in series, and leaving the electric tank ON (but with it's thermostat set lower than the aquastat on the indirect) is probably the best of all worlds. Your hot water will never be colder than the electric tank's thermostat, but as long as your hot water volume has reasonable daily volumes during the heating-season, the duty-cycle on the electric tank will be next to nothing (if not actually zero, which it might be). Then, when you turn the boiler off for the season you don't have to do anything more- the electric tank will still be on.
If you want to cut down on the electric tank's standby losses even further (to ensure it NEVER fires during the heating season and doesn't add much to the standby losses of the indirect+ electric since they'll both be hot during the heating season), 2" of foam insulation (bead-board styrofoam or XPS pink/blue/green board) under the bottom and on the top, and an R11-R13 fiberglass wrap around the rest approximately doubles the insulation of a typical tank, cutting it's standby loss in half.
While you're hacking away on the plumbing, depending on your drain & boiler/water-heater layout it might make $en$e to spring for a drainwater heat exchanger on the main shower to the house to preheat the cold water feeding both the water-heater chain & the cold feed to at least the shower, if not the whole house. They recover ~50% of the heat used during a shower, which improves the apparent capacity &/or recovery time of the tank. (This works for extending hot water heating capacity for showers, but not much for tub baths, since the drain flow has to occur simultaneously with the potable water flow to get the maximum heat-transfer back into the incoming water). Electricity in most places isn't cheap, and oil, well... it's all over the place, but I don't expect it to stay at current prices for the next decade (do you?)
These suckers cost ~$500-700, but are a reasonable DIY project, and the return for 3-4 shower/day use is worth on the order of ~100 gallons of oil/year (if all your HW was from oil, burned at 85% efficiency) or ~3000kwh/year (if all HW heating is electric.) It'll vary based on your incoming water temp from the street & the amount of shower-water actually used. In high-priced electricity markets like CT or NY, payback is typically under 3 years for all-electric water heating situations. Some people with electric tanks install them just to be able to take back-to-back showers with a little something left over for the 3rd in line.
See:
independent performance testing
one vendors' installation instructions