Thanks!
Just going to summarize what I'm doing in case it's helpful to someone else and post some pics when I'm all done. Pretty sure this will work. Thanks for all the input.
I got plus nuts:
https://www.amazon.com/dp/ (commissions earned)
Will need to stop by the local store for the appropriate 1/4" bolts and open up each hole in the track a tiny bit. The M6 Hardware that comes with the set will be stored away with the millions of other stored hardware for a yard sale after I'm dead.
Flat-head 1/4-20 stainless steel bolts strongest possible should be easy to get at the hardware store. I'm going to get the highest grade possible that will fit in the track and allow the towers to slide in.
A 3/8 drill bit should come close enough for making the hole for the plusnut. Make sure to get center-punches and a couple of starter drill bits that don't walk all over.
The CX-5 roof is stainless steel, so, we should not have dissimilar metals causing a problem so long the bolt and plusnut are both stainless steel. The track is aluminum but whatever since that is replaceable. I'm going to use a gutter seamer for the plusnut-roof contact, and antiseize for all other mating surfaces.
Amerimax Home Products - Seamermate 1 oz. Gutter Sealant - Tri-polymer sealant for outdoor use to seal gutter connections. Self-leveling. Paintable and stainable. Heat and fire resistant. Mildew and moisture
www.homedepot.com
https://www.amazon.com/dp/ (commissions earned)
Tracks and rails:
https://www.etrailer.com/p-Y01-120B-NT.html
Yakima and Thule make comparable products. Supposedly all hardware-track combinations between the 3 brands are interchangeable. You can use rhino track and thule towers for example.
I went with these because:
(a) strong, flat (not bowed) crossbars, these are rated at 225. Yes, I'm aware the setup is as strong as its weakest point. I'll try to install the tracks as close as possible to the side of the roof.
(b) easy to remove and move around
(c) don't care about aero since they can be taken off
This is useful:
www.rackattack.com
I considered:
OEM-style flush rails:
2017-2020 Mazda CX-5 0000-8L-R07 - 2017-2020 Mazda CX-5 Roof Rack Cross Bars OEM 0000-8L-R07 | BAM Wholesale Parts
www.bamwholesaleparts.com
but it wouldn't provide me with the flexibility I want.
Raised rails:
OMAC USA Top Roof Rack Side Rails Bars Aluminum 2 Pieces Black For Mazda CX5 2017-2020 | Compatibility; Make: Mazda Model: CX5 Years: 2017 2018 2019 2020 | Omac Shop Usa - Auto Accessories
omacshop.com
Just don't trust it.