I wonder if it wouldn't be possible to add grain filler now, followed by
toner
and a few light finish lacquer coats.

In rec.antiques.radio+phono, there was a recent conversation about a clear
wood filler (Crystalac). See "Wood Grain Filler" from Jan. 13th.

One guy claims that it works well, although he hasn't used it over an
existing finish, as you're thinking.

Might be worth experimenting, although if it took multiple tries & lots of
fiddling, it may be simpler to remove this finish and start over.

Phil Nelson