To heat a tiny place with a reasonably open floor plan, the mini-split plus radiant cove heaters is probably the right approach, and it will cost half or less what it costs to heat with best-efficiency oil boilers (even at N-Star or Nat'l Grid pricing), at well under half the installed cost.
As you've discovered, there is no oil-boiler made that wouldn't be at least 3-4x oversized for your actual heat load. Mini-splits are amazingly quiet, comfortable and efficient, if you size it correctly then "set & forget", letting it modulate with the load. Radiant cove heaters are also quite comfortable even when the room isn't yet up to temp, because they work by heating up the objects in the room directly (including the humans), not the air. With the doors open to the larger open space heated by the mini-split the temperature delta stays pretty small, unless you have a lot of window area or mediocre insulation in the exterior walls of that room. With the mini-split supplying the bulk of the heat, and letting the other rooms lag a bit when unoccupied it stays comfortable without using a gazillion kwh of electricity. (I have relative living in a really poorly insulated place a bit bigger than yours who heats with a 1.5 ton Mitsubishi, and uses electric baseboards in the master-bedroom only when the temps drop below 25F, which in her case isn't every day, otherwise I'd set her up with cove heaters.)
The 15,000 BTU/hr probably a bit higher than the actual heat load, may even be on the high side, but simply multiplying by the length of the baseboard isn't going to tell you much. If you have a regular oil-filling service and they stamp a "K-factor" on the slips you have enough information to get there.
If you heat the main rooms with the mini-split, and temperature-balance the doored off spaces with radiant cove heaters you'll be golden. With a room-by-room heat load calculation you can size the cove-heaters reasonably, and at 600W or less they're easy to control via a combination of occupancy sensor & line-voltage thermostat so that they only come on when the room is occupied. (A manual override switch for the occupancy sensor for use in bedrooms while sleeping is easy to deal with.)
"Well insulated" isn't a well defined description- what is the construction of the wall assemblies layer by layer, including the cavity insulation amount/type, the window U-factors (or describe them, if you're not sure), and the amount/type of insulation in the attic? (Describe how the crawlspace & basement walls are insulated too.)
Got a ZIP code? (For fine tuning the outside design temps, etc.)
A propane fired condensing hot water heater (AO Smith Vertex) and low-temp panel radiators with room-by-room micro-zoning could get you there too, but at 2.5-3x the operating cost of heating with mini-splits & occupancy-sensor limited cove heaters, and 2x the installation cost of the all-electric solution. But it's better to just get out of the volatility of the fossil-liquids markets- the price trends aren't looking all that good, and a the high cost of new-production oil/gas it'll clearly never go back to the relative pricing of 1970, or even Y2K. The current low price of natural gas is only due to the high prices they get out of the liquid fractions (like propane) out of those fracked wells. Fracked dry-gas is a serious money loser at the recent years' price averages. Even though it came out of the same holes in the ground, you haven't exactly seen propane prices falling with the newer extraction techniques.
[edited to add]
You live in a 6500-7000 heating degree day (base 65F) climate. At 600 gallons a year in an 85% burner that implies (worst case) 600/6500= 0.092 gallons per heating degree-day.
That means you're consuming 138,000 x 0.092= 12,696 BTU per HDD source fuel...
...and 0.85 x 12,696= 10,792 BTU/HDD as heat that went into the system.
With 24 hours in a day, that's 450BTU per degree hour (base 65F).
Central MA 99% design temps run between 0F and +5F, call it 0F, and you have 65F-0F= 65F heating-degrees.
With 65F heating degrees, at 450BTU/degree-hour, your implied heat load is 65F x 450= 29,250 BTU/hr.
That's a lot bigger number than 15K, but also a very big number for a tight well insulated 720' house.
But if you're using the boiler for domestic hot water with an embedded tankless coil in the boiler it skews those number considerably, since it burns a lot of oil in idle mode during the 4 months of non-heating season, which is why a K-factor on a mid or late-winter fill up would be useful.