any advice?
well, since you asked, - yes - too fine of a thread to hold fast in the bracing..
your options are: find a more pronounced thread fastener.
same with drywall screws - there are fine thread and coarse thread screws.
the fine thread will not hold nearly as well as the coarse thread.
What I used was flat head stainless sheet metal screws...... no worries.
available at https://www.BoltDepot.com or your Big Box Hardware Stores.
18-8 x 1-1/4" 316 Stainless Steel.
the ones that are not self-tapping - pilot holes must be drilled.
a little more time consuming, but, are worth the extra trouble.
If you prefer the self tapping, they are available (for a little more $$).
Question: you said you were using aluminum self-drilling screws.
are the screws actually aluminum ?
- or - are they zinc/paint coated steel screws for use on aluminum products
such as gutter downspouts, PVC fence, etc.
.