Stop. Buying. Electrical gear. From. Amazon.
It’s all nasty cheap junk that will kill you. It doesn’t even begin to comply with US safety codes, and is not certified by an independent lab, as required by law. (Any state which as adopted NEC).
Here’s a newsflash about Amazon: Jeff Bezos believes in open markets at all costs. Amazon has internal server farms, warehousing, shipping and of course a retail web site. Bezos has opened all of them to public use - via AWS, Amazon Warehusing, Amazon Fulfillment, and your undoing: the Amazon Marketplace.
You already know to avoid some channels, because what they sell is complete and total junk... right? It’s mail order, and it takes advantage of poor coverage by US Customs to sneak things through that don’t meet US safety standards. Well, all those same sellers jumped onto the Amazon Marketplace. Everything that does not say “Ships from and sold by Amazon.com” under the Buy button, that is eBay-tier trash-ola. It ships via Prime because Amazon opened that up too.
Even things sold by Amazon are corrupt due to (one guess) “open inventory bins”, where any third party seller can inject counterfeits into the inventory bins Amazon ships “their own” items from. /facepalm
The upshot is that Buying from Amazon is so problematic that it is simply not a safe place to buy anything that connects to AC mains power.
No more of that. For ordinary stuff, your local electrical supply house. For exotica like variacs, Digi-Key, Mouser, Grainger, Galco, or McMaster-Carr.
This is a $100 solution to a $10 problem
I don’t know why you’re brain-blocked on just buying another heater. But electric heaters are the cheapest (by which I mean inherently inexpensive due to their nature, not junk) appliances in the world. Resistors are cheap.
So if you want an “intermediate” heater, simply get another one of intermediate size, or with an adjustment knob. This is the point where I note that a 240V heater, run on 120V, will give 1/4 the heat.
Get rid of the fan. Now you can use $10 solutions.
Further, removing the “fan/rotate” component will be a huge step in the right direction. (That is a motor load, that requires particular regulation - heck a variac isn’t even the right thing; you’d want a VFD - and if you think variacs are expensive, lookout!) With no motor loads, the thing is simply a resistor, and will behave in a docile manner using a variety of energy-control devices (including a VFD), not least, a plain old cheapie leading-edge triac incandescent “dimmer”. (Incandescent lights act like resistors once they’re lit, so you’re copacetic.)
Or hack it for separate control
If you feel a huge, compelling need to keep the fan but regulate the heater, then simple matter: hack the device to completely separate fan control from heater control (so no wires at all connect them), and add a second line cord, one feeding fan, one feeding heater. Two power cords is allowed as long as there is 100% separation of the wires, so nothing connects power cord 1 to power cord 2. (The third-prong safety ground is allowed to connect) Now you can put the heater section on any of a variety of dimming devices.
Another example of “2 line cords” is a 4-lamp fluorescent light, wired so there are 2 switches and each pair of tubes can be turned on separately for different light levels. They are often cord-and-plug connected in the ceiling.