I'm new to these Mazda trans, also, but from what I've read (because mine is obviously about to expire) the 5th/reverse synchro is a weak spot, or at least goes bad frequently enough that it's a frequent topic. That will cause your 5th to grind going in.
Check with rboyer on this forum - after lots of fruitless searching for something that I could have some reason to have confidence in, I ended up getting one from him. His price was very competitive and included shipping. He even takes them partly apart and inspects them and makes any repairs necessary and has a guarantee. He has 2 levels - of course with 2 different prices but both are very reasonable for what you get. The more expensive one is taken totally apart, not just the major "clusters" out, and has a 6 month warranty. And is still very reasonable compared to other prices I've found.
I don't have mine yet, am expecting it maybe this week.