Great I am glad you solved the puzzle.
In my opinion a likely cause for so much build up may be the back pressure on the system due to the long vent run with so many elbows. The belt driven fans as you have are not as powerful as the fans connected directly to the motor.
You will likely run into this problem again in a year.
Keep in mind the lint build up presents not only a drying issue but also presents a fire hazard. Hence the reason for the building codes mentioned earlier.
Installing the vent booster should help prevent the buildup of lint and prevent downtime, overheating of the unit, potential broken parts (i.e. heating element, thermostats, etc) increase drying efficiency thus reducing drying time saving on electric cost and as stated before reduce fire hazard.
Oh BTW I'm a retired fireman. Partly reason for entering this business.

The Fantech 110 is the most common one used in my location. We do not carry vent boosters here @ APP that I have been able to find.
I would make sure whichever you purchase that it is UL listed and approved for the job.
The Fantech models have a 4" inlet and outlet and will hook right in line as you stated at one of the connections.