First i would do is to make a graph out of the power-sensor from your Aqara-plug ( Grafana make detailed cool graphs and with InFluxDB you will have this as long-term statistics, thou you might wanna use apex-graph-card( then your entirely depended on the homeassistant_db “Purge-Period”.
Secondly either add a thress-hold to the the graph-card, as an Attributes, or you are maybe able to use the Threshold-integration, to read direct from your power-sensor.
Or better of a Template-Sensor to read the power-sensor (i.e When power-usage goes from 0 to >0 start heating-period, when power-usage goes from >0 to 0 stop-period … not sure this exact example will work, but there is help here, for exact “definition/syntax”
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After monitoring the graph and your heating system. i.e before you light the fire, and after starting the Fan, you will see whats happening with the power-usage, and from that decide when/what level to set the lower-power-usage-threshold ( could be zero Wat, or 0.07 Wat(on the external-plug), before fan starts, and then immediately when it starts i.e 8 Wat ( this is where your time period “Fan” starts) ( when power goes from zero(something) to +(above something)
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When the Fan stops( you can also see that on the power-graph ) , but it might use less and less Wat before it stops / as it also use more and more Wat, when the heat goes up.
As im not sure how accurate the plug is, or if it +boilers internal electricity use just a little, even before you turn on the switch on your boiler, you have to figure this out, as well as exactly how your Fan acts during heating period ( i.e does it stop periodically )
Normally i would have stopped the Fan, most likely before the “ashes state” as you mention
The “normal” drag would be enough for the last “glowing almost ashes” to burn out … im not familiar with your wood-boiler, chimney etc , neither the exact function of the internal fan. … however you will notice on the power-graph, that the fan will use less and less power ( unless it’s some old home hack, that runs full power from start to stop, which i doubt ) 
… so know your “Devices” is AO , to figure out best/cheapest/effective solution
Lets say, over a week, you see on the power-graph , that the Fan uses to go down to a certain level i.e 4 Wat, or slowly goes further and further down, to eventually zero, or zero+ etc. “sleepmode-power-usage” , you decide a "braking-point " where your Fan stops ( when power goes from +something to zero-something)
You can direct read this from the graph, and obviously above is not an optimal solution, but quit accurate, best would be if you attach a “sensor/switch” direct to the Boilers Fan-switch ( i would/have ), unless ur absolutely sure it dont use any power, when the Fan is turned of, and that the Fan don’t periodically stops during “heating-period” , this you will also notice in your graph)
PS: if it’s just for “monitoring purpose” i.e not planning any advance automatons attached, triggered from the Graph, i would go for Grafana+InfluxDB , and an automation could easily be triggered from the power-sensor with template, even a template-counter
But start with a graph, and monitor your system, get to know the boiler/fan/power-usage pattern, you’ll get more insight and ideas within a month …And then search this forum for i.e “heating automation”
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