That coolness in the basement in summer is actually a mold risk to the back-side of the finish walls, since the temperature inside the studwall cavities can drop below the dew point of the ventilation air. That can be controlled with a dehumidifier, but a dehumidifier converts the latent heat in the water vapor into sensible-heat- hot air, raising the temperature of the basement in summer. Even without mechanical dehumidification amount of "free" cooling BTUs is pretty small too, typically less than 2 BTU per hour per square foot of floor.
But the amount of NOT so free heating BTUs of an insulated basement is substantially higher, since the uninsulated above grade portion of a poured concrete foundation is less than R2, more like R1-R1.5 unless it's unusually thick. Adding an empty 2x4 studwall with half-inch wallboard on the raises that to only about R4 "whole-wall", with the thermal bridging of the studs factored in. When it's 60F indoors and 10F outdoors (a typical coastal CT outside design temperature) you have a 50F difference, and every square foot of above grade R4 wall is losing 50F/R4= 12.5 BTU/hr. You probably have on the order of 300-500 square feet of just the above grade portion of the foundation, so it's representing a heat load at design condition of 3750- 6250 BTU/hr which is probably more than 10% of the whole house heat load, even though it's cool down there!
Even with R15 rigid foam insulated walls my ~1600' basement never gets over 70F in summer, and I still need to run a dehumidifier during muggy weather, but it's duty cycle dropped by about half (!), primarily due to better air tightness. In mid winter it never drops below 66F, heated only by the distribution & standby losses of the heating system, though in spring it'll sometimes hit as low as 63-64F, since the ground is still pretty cold, but the heating system's duty cycle (and subsequent losses) drop to near-zero. By half past June it's back in the 66-67F range, and by September it MIGHT hit 70F. At my house insulating the foundation took about 20% off the fuel use per heating degree-day, while raising the average wintertime temperature in the basement by about 3F, (65-66F instead of 62-63F) and raising the coldest day temp in the basement by about 10 degrees. YMMV.
It's hard to make a case for ripping apart a recently built finished interior just to insulate the foundation, but if you have access to the foundation sill and band joists it's DEFINITELY worth air sealing and insulating those. But it's also worth insulating the foundation in the unfinished areas. There are several threads that cover how to do that on the cheap without turning it in to a mold farm on this forum. In southern New England there are several
vendors of reclaimed rigid foam that can make it pretty inexpensive, but there are several details that need to be heeded to keep it mold-free, even with foam.
If you insulate the basement ceiling to a code-min R30 and air seal the subfloor to limit convection between the basement and first floor it'll lower the heat load, but it'll also lower the temperature in the basement making it less comfortable (unless actively heated) and put the boiler on the opposite side of the insulation from the conditioned space, where the standby & distribution losses accrue to the basement only, with at best minimal savings. It's almost always better with retrofits to just insulate the basement walls, so that it's all inside of conditioned space.
A reasonably tight 1200' house with some attic insulation and all of the above grade walls with at least R11s in the studwalls, and NO foundation insulation will usually come in around 23-25,000 BTU/hr @ 0F (cooler than the 99% design temp or most of CT). Insulating the foundation usually drops that to 20,000 BTU/hr or less, while improving the comfort levels in the basement. With a walk-out basement or drive-under garage those numbers will shift around a bit, with somewhat higher heat loads, but not dramatically higher. Your true heat load is almost surely under 30,000 BTU/hr (or very likely would be with a few upgrades to the house), which makes all but the very smallest cast iron boilers oversized. Even the tiny down-fired 3 plate
W-M CGa-2.5 has a DOE output of 44,000 BTU/hr, which would be fine if the load is actually 30K, but if it's much under 25K the duty cycle would be low enough that it might not hit it's AFUE numbers. (AFUE is tested at a 1.7x oversize factor.)
I'll be curious to see where the fuel use heat load numbers come out, and what the DOE output of your CGM Series 10 is rated. If it's ridiculously oversized but you're still going to keep it, a heat purging economizer such as an Intellicon HW+ can knock a double-digit percentage off the fuel use. They work by inhibiting the burner on an initial call for heat until the boiler temperture drops to it's low limit, and "learns" the system behavior by monitoring burn cycles, and cuts out the burner ahead of the anticipated end of the call for heat. That lowers the standby temperature of the boiler, lowering the standby losses, and lowers the overall average operating temperature, lowering the distribution losses. The latest-greatest versions run about $250, but you can find older versions around for under $150 if you scrounge. A decade ago it was called the Intellicon 3250 HW+, and was also sold as the Beckett Heat Manager. There are newer-fancier products of this ilk from multiple vendors, but any heat-purging economizer will do pretty much the same thing.
