The choice of thread is not so much what the final product is, rather it's about the weight/thickness of the fabric used.
The
Gütermann website tells me that SewAll is suitable for most fabrics and should be used with a 70, 80 or 90 needle. Their Extra Strong (which is what I think you mean by "upholstery thread") is suitable for heavy woven fabrics such as thick denim or canvas or leather, and that it requires a 90, 100 or 110 needle. If you go for the thicker thread remember that you should use a longer stitch length, maybe 4mm.
You say that the cushion seams are under a lot of pressure when removing the covers. Do those covers go directly over foam? When I made a piped box cushion a few months ago DD told me to make an inner lining using something slippery to make getting the top cover on and off a lot easier. I used a bit of poly satin that had been in stash for ages, nothing fancy, no seams neatened inside but the last side did have to be closed by hand but getting the top cover on over it was so much easier than getting the foam into that liner.
Also you need to have the zip long enough to go right along the back, around two corners and a couple of inches along each side. Put the zip into the middle of the back section, don't try to get it next to the piping!