Eco4 was designed to be a whole house solution to improving energy efficiency and lowering bills. You can not combine it with the Boiler Upgrade Scheme, or similar grants. If you are not able to receive full funding for your work through Eco4 though, and you are in Wales, you might want to look at the Green Homes Wales scheme.
GHW offers a 10 year interest free loan, which can be used to bridge any gap between ECO4 funding, and the cost of delivery. It could also be helpful if you would a solar battery (which isn’t covered in Eco4). Repaying a battery over 10 years could end up being a great investment, but speak to your surveyor about the specifics of your house.