Thread Durability

I'm looking to sew more canvas coverings for yurts and other semi-permanent structures.

Five years ago I made my first yurt cover. I searched around for the best natural fibre to use for thread, but the advice I got was that any natural thread would have me resewing my work every few years. I went with some #138 thread. I figure like the billboard plastic liner in a wofati, a relatively small concession to the gick economy was worth the larger benefits. After exposure to the full seasonal cycles of northern Wisconsin all these years, the cover is holding and I don't see even early signs of failure.

I recently started sewing another tarp, but couldn't locate the spool of #138. I had purchases a smaller spool of #69 at some point and just started with that. When it occurred to me I was going to be putting a lot of faith in this tarp holding up to the weather I began to question whether #69 was enough. That was the thread used and recommended by the offgridpermaculture website I used as my initial guide, but that website proved to have enough other fundamental flaws I'm not going to assume it was the best advice.

Looking on Amazon, #138 has gone up since I purchased it last time and is around 3 or 4 times the price of #69. Does anyone have reliable experience comparing the performance of these various threads under harsh conditions?

I'm using 12oz duck canvas, planning to move up to 18oz after my current supply is exhausted. While the thread is holding, the 12oz has started to go threadbare in spots...

Generally, the thread lasts longest if it matches the material in fibre content and weight.

Too light, the thread breaks (this is my preferred failure point)

Thread too strong, the fabric breaks (so much harder to repair)

The type of seam seems to matter more than the thread

In tent construction, i find synthetic fabric decays in the sun much faster than natural.  I mostly used cotton duck (a bit thicker than jeans) with thick cotton thread like top stitching for jeans.  

I do a french seam.  The thread from both parts of the seam are inside, not exposed to weather.  If the seam takes a lot of movement or change (like near a door), I tend to top stitch it down.

The less the seam has to move, the less it seems likely to break...that said, it also depends on the grain of the fabric.   I made my triangles so that a bias cut edge always sewed onto a warp (long) grain edge.  Fabric is flexible, especially on the bias.  Thread isn't.