Your observations about the fluid viscosity are spot-on IMHO. I have experienced the exact same thing, and had to make some minor adjustments to my "mod" to accommodate.
I don't use the straw and BP system. Instead I'm a "spring and blue pad" guy. I have my reasons. Anyway, my daily vape is ECOpure based... which is a fairly viscous VG liquid. If I get my cartridge set up perfectly for the ECOpure, thinner, watery PG based liquids will run right through it.. flooding the atty.
For the PG and thinner liquids, I have a cart set up with a spring that is about 1mm longer (vertical springs). That causes the pad to be compressed a bit more when it contacts the bridge, and that closes up the pores in the pad.... And then it works wonderfully for the less viscous liquids.
I wish I had found the "one size fits all solution"... but sadly that hasn't come my way yet. So, for the time being, I just have two different cart varieties... "thick and thin".
P.S. Of course.. the other alternative is to either "thicken up" my PG liquids, or "thin out" my ECOpure. That works equally as well.