Sounds like the flat plate isn't the pinch point. It's flow, probably on the incoming line from the owb. I'm assuming that pump is OK? Like a broke impeller, etc.
My thought is- The options are larger underground lines, double pumping, or more emitters in the house so you can heat with lower temp water. I usually favor the extra emitter route. A kick space heater, some Pex under the floor, custom taller baseboard covers to draw more air... Lots of options.
Had one like this last year, eventually found there was a bypass between supply and return on the oil boiler that was left open. Also the temp probe on the Owb was reading about 20 degrees low. Fixed both and problems disappeared.