I have gotten used to the corner being filled, so I wanted to keep the
giant, but just reduce its height. I can't imagine a small one being
attractive in that tank, so thanks, but I will try planting the top
half.

No reason why that won't work fine

Since I have loads of roots hanging from the middle, I expect I
will bury the stem and as many roots as I can.

Don't bury the stem - just bury the roots a bit. Just weight the thing down
using plant weights or tie it to a log or a rock with either thread or
fishing line till it grabs hold of the rock/log of its own accord.

Or, would I get better results for the 10 gallon tank if I cut midway and
leave the bottom
minus large older leaves? Is this giant going to remain a giant after
separation?

It should grow again.

I want to keep the corner full and high. I don't want
the stem to not grow again. Hope I am making sense.

It will shoot again from below where you cut

Thanks for someone for mentioning the value of thinning. I like close
vegetation, one tank the growth of anubias is very heavy and leaves
are separating and floating to the top. I have anubias in 4 tanks and
this is the only one where the leaves are coming off the stem. Maybe
I need to thin some?

I find that if I don't thin some of the leaves also get stunted. Where I
thin my AnnubiasI tend to get bigger and healthier looking leaves.

HTH

I.