On the plus side, no matter how hard somebody tries it won't be easy for them to see both sides of the neck at the same time to compare the color.
That's something that you're aware of. So it'll be more noticeable to you than it would to someone else.
If you end up deciding to go down to the bare wood on the back of the neck though, I'd do the whole back of the neck.
Then you can use whatever finish you prefer & should be able to get the whole back of the neck looking the same.
There are wipe on poly finishes that go on easily. I don't know how well those would hold up on a neck though.
You can also use spray poly or lacquer like Deft. And you an buy spray tint to get the vintage amber color if you want to.
Don't use an oil based wood stain under a lacquer if you're gonna sand it smooth because the solvents in the lacquer will cause the stain to bleed into the finish coat & it'll sand through.
But I've done a couple of necks using an oil based Colonial Maple stain to tint the wood, followed by a boiled linseed oil finish. And once it dries it gives a smooth finish that isn't sticky or glossy and plays IMO better than anything else I've tried.
What I mean is, sometimes I just don't know what I mean. Ya know what I mean?