Regarding ventilation, pressure

A question from a fellow grower:

I do have a query regarding ventilation.

My 1.2 x 1.2 x 2m tent is in a room with an en suite. My ideal external access is via the extraction fan in this adjacent shower room. This fan is disconnected and linked to 100mm ducting which goes through a small attic
and out the roof (only about one metre travel).

As the fan is disconnected I can flow air in either direction. Trouble is, where the ducting meets the roof, it is covered by a kind of hood like tile to keep out rain, which only has two small apertures for the escape of air. Not a problem for the existing low power bathroom extractor, but too small I fear for my RVK to vent the tent without causing too much back pressure. (RVK 100mm diameter 175 m³/hour)

So I was wondering; what if I duct my tent intake to this instead, and vent in fresh air passively via negative pressure from the RVK extracting into the surrounding house?

See, I always thought it vital to exhaust externally. So my question is, is that so?

I can have a window slightly ajar but would rather not have it open 24/7 as it will look peculier with the curtains shut.

Or do I just go ahead and use the roof holes to exhaust, maybe see about making them bigger?

Also, regarding pressure, would using 125 to 150 mm ducting on my 100mm RVK reduce resistance and move air more freely?

Oh, and one other thing; I have two of the RVK fans, so I was thinking either running two on light and one on dark, or one always exhausting with one intaking on semi power using a variable voltage plug.

Or something different altogether maybe…

Anyway, thanks for your time and effort, much appreciated.