To be fair, though, space heaters are dangerous in older homes with unknown electrical system quality. You're running a circuit at just shy of it's max load for potentially long stretches of time. Any poor connections (cough, backstabbed outlets) along the circuit or damaged wiring could result in a fire.

Aux heat for heat pumps (not mini splits) usually starts at 5KW (4 space heaters) and is a dedicated circuit direct to the panel and sized correctly. It's also contained in a steel enclosure with strict rules about distances to flammable items.

The DIY Mr Cool has an extra 3kw heat option for colder climates. Not sure how well it works.