Adding soil, especially to that level, will kill the tree

The root collar must be the same height. You can put a ring around it and fill behind it but you still need access to the trunk to clean out mouse hiding leaves.

NO increase in soil level!! As little as a 2" increase in soil level surrounding the tree will effectively smother the root system and kill the tree, The effect is not necessarily immediate but none the less inevitable.

Don't raise the soil level, like others have said here, you need to keep the root collar exposed or you can smother the tree. What I might do is what poncirusguy suggested, or, you could get some nice inconspicuous pots and plant a grass or tall plant in them to place behind the tree to give the illusion that it's taller. Or plant some vining plants. Just something to obscure the back wall of the "well" around the tree so that it looks less conspicuous. OR, you could illuminate the tree, which will give you a nice play of light and some optical illusions if you do it right.

Thank you for adding the clarification, I don't usually say more than don't cover the root collar/flare (depending on the person I've heard both), since, unless you're getting really creative, I don't see how you would keep the flare clear and cover the roots.

Branch collar. Root flare. 😁

It would be thoughtful to give the tree a couple inches of mulch.

It looks absolutely fine!

I mean, it looks like it has tried to escape in the past and you put an end to that, but it looks good.