On the SEU issue I’ll add in that even in LEO you can still get SEUs - the ISS is in LEO and gets SEUs on occasion. There’s also the South Atlantic Anomaly where spacecraft in LEO see a higher number of SEUs.
>vacuum is a fucking terrible heat convector
Yes we're talking about radiating not convection
And a kilowatt from one square meter is awful. You can do far more than that with access to an atmosphere, never mind water.