Get another 2000W unit. Seriously.
Place it where practicable and useful, and place it in series with the existing 2000W unit.
Together they are now individually 500W units, and 1000W together. I believe that's the number you wanted to hit.
I mention this because resistive heaters are naturally perfectly inexpensive. Talking about for-permanent-installation baseboard heaters, here in the US a 2000W unit is a mere $50+tax. So this is a very economical way to do what you want, assuming there is not some mad discrepancy between units.
The gory details
When doing series connections with commercial products, I prefer to match up same-size.
Now, if you want all the numbers, here they are.
Voltage Power Current Resistance Conductance
E or V P I R G
240 V 2000 W 8.33 A 28.8 Ω .0347 Siemens
240 V 1200 W 5.00 A 48.0 Ω .0208 S
240 V 800 W 3.33 A 72.0 Ω .0139 S
240 V 1000 W 4.17 A 57.6 Ω .0694 S
120 V 500 W 4.17 A 28.8 Ω .0347 S
240 V 750 W 3.13 A 76.8 Ω .0130 S
240 V 3200 W 13.33 A 18.0 Ω .0556 S practical circuit limit
240 V 3840 W 16.00 A 15.0 Ω .0667 S absolute circuit limit
When placing units in series, add resistances.
Your calculation for the 2000+1200W heater in series is correct: 28.8 + 48 ohms = 76.8 ohms giving 750W.
All three heaters together are 16.6 amps. The change I suggest will drop this to 12.5 amps, giving 3000W of heat in the room. This is within the 13.33A circuit limit after the 125% derate for heaters is being applied. Your country may not require this derate, but apparently, your fuse does.