I’ve built an quite large bungalow type T-shaped house in 2008 The T is 20m by 22m and there are 5 wc’s.
I’ve have a vent on the 3000l septic and 1 11/2” went up to the roof in each end of the 4” main tubing that connects from the toilets to the septic.
Normally no or very little smell in the bathrooms, but sometimes was very heavy smell when the wind comming from a special direction
Found out that the smell came from leaking connections between the toilet bowl and the 4” pipe in the floor, You can’t normally seal that from the outside as there is vent holes on the back side of covered bowls you can’t reach.
Checking in the building warehouses there are almost no hardware to do that seal. Only some rings of soft clay type to put there and lower the bowl and hope the clay will be pressed out and fill out the space airtight to the floor…….?
That is very difficult to lower the bowl straight down on the clay ring and not disturb or deform the ring.
Instead of reinstalling the 5 toilet bowls I put a small 60mm 12V type Pabst ball-bearing axial computer fan in one of the 2” end-vent tubes, that sucks air out of the tubings. These type of fans are really low cost and runs for years wit no problems.
I mounted it just with duct tape (Thaiwatsodu) and connected to an old 12V mains adapter. It draws only 0.7Watt
I then cut and restricted the 2” area to 1/2” in the other end of the toilet waste went, to keep a slight under-pressure in the system.
This totally and finally solved the smell problems from the toilets in all 5 bathrooms at the same time.
I then made the same trick to the vertical vent from where the drain and sink pipe go together to the concrete septic and put a 3” water lock to the pipe entering that septic (no2 after the black water septic from the toilets)
Thi also slowly sucked the smelling air out of the drain system and floor water locks (the thai ones on the picture in this thread dries out in a few hours)
Good luck with your other solutions, but these small fans really made it!