It's the design of the spool that makes them weak - the "carrier" in the spool has a bunch less material.
This can be combated in a 9" with a load bolt 3rd member. Assuming you're using 35 spline stuff, if you run anything other than a spill you'll be in aftermarket 3.25" carrier bearing race OD center section land anyhow, though the load bolt 3rds add more cost than a grizzly or Detroit would.
The 3rd pinion bearing helps keep the pinion rigid, but makes the carrier smaller on the far end from the ring gear in all 9" cases.
But also recognize that with ~2:1 gearing after the R+P, the deflection will be a lot less.
Sean With how you have described your use, I think any 9" carrier wouldn't be a problem.
My experience- I've run 5.13 gears with no portals on sticky 40's with water with 9" with spools, Detroit, and zip lockers. I broke one rear ring and pinion really Launching and landing on the throttle with the Zip, but something was going to go in that scenario. Otherwise, 10+ trouble free years with no portals, and portals would just make that load less.