I have always had the best luck with them in protected shade with no real direct sun. Perhaps the tree removed was a bit of a screen for yours. You can see if it will adapt to the change and fertilize with something like fish emulsion or move it to a spot that's a bit more shaded. Just my thoughts. They are such beautiful trees. I used to grown them in clusters of three inside the house in a bay window. They flowered all year long. Hence the name from the Victorian era of parlor maples.

If it's one you really like you could root a cutting before removing or attempting to relocate. I am dubious if one that old will survive a move.
Mine always needed partial shade too. Good luck.

What about just cutting it to the ground and letting it re- sprout? I used to periodically cut out large vertical stems because they would get infested with scale, new stems were always coming up.

They were able to handle full sun in both locations I have lived, but I also did use them in quite shaded areas. However, I am not familiar with "Niobe."

I live in san Francisco and mine only like full sun. I think full sun will work, they need more water and better soil, compost, compost, compost and great drainage to live will.
Yellow leaves does sound like over watering however. Do you have clay soil? They do not like clay soil.