No, you can't do that using Sonoff since it is an on-off switch with relay.
You need an Triac to do that. Normally the brightness control applications for AC will be done through Triac by changing the firing angle.
Given the basic requirements, you might want to try out mongoose OS. It's fast to setup, support for Amazon Alexa, Google home etc. Lots of documentation available.
The only thing I'm not sure of "out of the box" is your act as an AP requirement for setup.
Mongoose OS is Open Source and dual-licensed: Mongoose OS Community Edition - Apache License Version ...
You can certainly make a wifi controlled PWM fan with an arduino and a wifi module. It would be fairly easy to throw together and would probably be a fun and rewarding project. All though if you don't want to do it yourself, I would recommend looking up DC motor controllers. Those are all PWM and are fairly inexpensive. You can get one with the voltage you ...
I've struggled with the same this morning, and finally managed to enter DYI mode following these steps:
Start here with 3.5
On the Sonoff device press button for 5 seconds (light pattern changes)
Press button again for 5secs (light pattern changes again)
Connect to the isteadxxxx AP using pwd 12345678
Browse to http://10.10.7.1
Enter your SSID ...
Oh no. Sorry this happened !
I would try to erase the flash and format the file system and then try flashing it again.
See: (gotchas and the upgrading firmware pieces)
On the endpoint. They get their time from the net and timers continue to work without the net. Even after reflashing the firmware (for example with Tasmota) the endpoint retains the ability to store timers and obtain the time from the net.
Absolutely not. The SonOff Mini does not have a transformer power supply, and the 'switch' contacts are at half mains potential when working. This has the unfortunate side effect that if the antenna is not well insulated, a short to the antenna seems to be able to destroy the device (I assume it was a ground short, but it was in the ceiling void). Replacing ...
There are devices made specially for dimming using only on-off impulses to set the brightness. A common pattern is to use a short on/off signal for on/off, and a longer on/off signal to increase brightness in n % steps.
So provided you can make the Sonoff switch on and off sufficiently fast, and reliably to get the timing right, adding a device like this or ...