I would consider a 200 to 250,000 btu/hr heater to reduce the output temperature.
For example, at 40 gpm for a 400,000 btu/hr heater results in a 16.8 degree temperature rise from inlet to outlet.
If the water going into the heater is 103 degrees, the water exiting the heater and coming out of the returns will be at 119.8 degrees.
Below is the flow in gpm (X-axis) vs. temperature rise (Y-axis).
You would need to get to 80 gpm to drop the temperature rise to 8.4 degrees.
A 200,000 btu/hr will have a temperature rise of 8.4 degrees at 40 gpm. 103 + 8.4 = 111.4 degrees coming out of the returns.
A 400,000 btu/hr heater will give you about 80 degrees per hour or about 1.3 degrees per minute. 25 degrees in 19 minutes.
A 200,000 btu/hr heater will give you about 40 degrees per hour or about 0.67 degrees per minute. 25 degrees in 38 minutes.