I have that a similar symptom of crusty scale buildup. I can get it to dissolve with CLR after evacuating all the water out of the bowl.
I also get ok results with Muriatic Acid. But that is a dangerous chemical not for use w/o major knowledge of chemical handling and protection.
But the scale comes back.
I have a different Toto back to back with that one than never develops scale. And an Am. Std. one that never develops scale.
I think that replacing is a good bet.
Many toilets available have a special finish available which can be called things like EverClean, or Cifonet and they are molecularly superior to plain porcelain, so consider that in your upgrade.
Toto as all mfrs now, has some good flushing toilets and some not so good.
MaP Testing site is the top resource for checking on good flushing, aside from Terry Love's excellent list.
The cheapest good flush around is Toto's Entrada. Best to buy from a local plumbing supply dealer.