Based on the new posts, still sounds like your copper coil inside your OWB that heats your domestic water is either partially plugged and you are not getting good flow and/or too much build up that you are not getting a good heat exchange into your domestic hot water flow. It might be time to disconnect your domestic water line to see what kind of flow you are getting thru it and if it is partially to mostly plugged, that is probably least difficult. I am guessing your OWB is not adding enough heat to your electric water heater and the electric water heater cannot keep up with the demand by itself. If you have good flow thru your domestic line thru your OWB coil, next would be to look at your copper coil inside the OWB and that involves unsealing the plate on the back of your OWB and pulling the coil out. As mentioned, there is probably some type of build up, probably scale, insulating your copper from allowing a good heat exchange to your domestic hot water flow. Cleaning the build up off would then be necessary.
If you put a HX inside the house off your heating line from the OWB, I would probably abandon or eliminate the domestic loop to the boiler and not have to maintain that entire 2nd loop and keep the domestic water system inside the house.
A late thought, does your OWB have a 2nd set of boiler water inlets/outlets that are capped? If you are willing to install new 3/4 lines, maybe you should install a 2nd loop of boiler lines and then put a new HX in the house. that still keeps all of your domestic hot water lines in the house. Just a thought?