So the humming noise got louder as power decreased.  It was quite a miserable humming sound at 5, bearable above 7, reminiscent of the back rooms.

The most efficient way to generate a quieter sine wave is an old fashioned autotransformer.  

https://www.amazon.com/YaeCCC-Transformer-Variable-Voltage-Regulator/dp/B076Y2XHRS/

The standard variac everyone has used for 100 years.  The homeless despot has a better deal. They're actually household items, not as cheap as a dimmer switch but far less than what lions expected.  It's actually the same price lions spent on the dimmer switch, relays, meanwell.  It would have just been the total price up front instead of incrementally.  It would have to go on the floor, with the user interface broken out to a separate panel.  This split arrangement would actually allow the front end to have a proper LCD.  It's quite an investment in space for just controlling a blower.  A common dimmer with all its humming was good enough for a much younger lion.

Commenters noted the variac hums at full load but no-one has ever captured the humming sound.  The blower wouldn't run them at full load.  Note if the input & output on a variac are swapped, it becomes a voltage booster.

Given the investment in speed controllers already equaling the cost of the blower, adding a variac would bring it to 2x the cost of the blower.  Thoughts turned back to developing a custom blower around a low voltage motor.