I haven't done the job you're referring to, but am familar with mineral scale, in some of it's various forms. That, plus one gentle but slow method to get rid of it. And a couple of hypothetical, more agressive methods.
You could start mild, & try heating
to as high a temp in it as you can. Changing the water frequently.
The idea being, that because distilled water is so "clean" it'll "pull" out/off some of the minerals when you heat it. Which is the reverse of what got you into that fix. Such being, that heating
standard water, with a fair mineral content to it, left behind said minerals, when you emptied the unit of hot water each time.
Plan B would be a mild acid. Like say Vinegar (Acetic Acid). I wouldn't heat that, but just let it soak, & see if the vinegar will eat the scale. As, for example, if you drop sea shells or similar (which I think are calcium carbonate) into Vinegar, you can watch them bubble & dissolve.
And as crazy as it sounds, Coca Cola might be another option. As it acts similarly to Vinegar, being an acid itself. Phosphoric I think, actually.
Also, assuming that it's not a vessel used to heat potable water, you might try CLR or orther commercial
, household, scale removers. Although what I'd be willing to use on it experimentally, would be based a lot on the replacement price
of the item.
As usual, the last resort - Try talking to the company which made it. AKA "the instruction manual" (which you likely can find for it online).