This doesn't have anything to do with saws, but it's too long for a comment.
Instead of trying to come up with a better stake, maybe come up with a
completely different solution.
My first thought at seeing the "penetrating hard clay soil" was to use a
wood/spade drill bit to drill a pilot hole in the ground before trying to tamp
in the stake. There are also wood augers that might work better, as they are
similar to the standard twist drills, except with a much larger groove for
removing material.
Spade bit:
Wood auger:
https://pjcplantservices.co.uk/wp-content/uploads/Wood-Augers-Bit2.jpg
I have a feeling that the stake won't maintain it's cutting edge with such a
long tip, so you might be hurting your changes, rather than helping, them by
making the angle more pronounced.
Of course, this depends on if you or someone else is driving the stakes in, as
well as if you have an extension cable long enough or a battery powered drill
powerful enough to do it.
Ok, so this part uses a table saw:
Using a sled, you can rig up a jig to allow you to make your 70 deg cuts. Even
radial arm saws might not have the travel you need to cut the 70 deg angle
you're looking for, but you shouldn't have a problem with the right sled on a
table saw.
Table saw sled:
Also:
You might want to invest in a fence post driver, since slinging a sledge hammer
at a 4' fence post isn't going to be easy. Also with them being wood, you're
likely to crack them with a sledge. A driver is likely to give you a centered
hit that is much less likely to split the wood.
Even if the stake splits/mushrooms the end, the driver will help you keep the
stake in one piece, since it would encircle the split wood, helping to prevent
it from splitting it further.
Believe me, a driver like this is much easier to handle than a sledge, in many
ways. Guaranteed hits, no bruised feet/shins/knees/ankles, no broken bones,
better balance, and they stay on the post when you need to take a break (just to
name a few benefits).
Fence post driver:
