The generator can be switched between 400V and 230V.
When looking at the generator specs, I was already wondering what that even means. 3-phase it outputs
is both 400V and 230V at the same time. 400V between phases, 230V between phase and neutral. So maybe it's just marketing speak and there is no switching of anything?
If there really is something to switch "between 400 and 230", it can only mean two things:
Going from 400/230 to 690/400, or to 230/132V. Latter would be extremely useful for your need. But see the manual if it explains what this voltage switching means. But I would put my money on their "voltage switching" claims being bogus.
One viable option, if you don't want to buy a new smaller heater, is to rewire two of the resistors in series and run it using a single phase:
8000W / 3 / 230V = 11.59A
230V/11.59A = 19.84 ohm (expected resistance of a single element, you can verify this with multimeter)
Two in series, current = 230V / (2*19.84ohm) = 5.80A, power = 5.80A*230V = 1334W
I'm quite sure keeping the circulation pump running and heating at 1300W is sufficient to prevent freezing. Unless you have some record-low -25degC sustained over a week or something.
And then again, if you do this, a smaller single phase generator would be well sufficient. Something rated to 2000W continuous.