How fortunate it was a phonetic script! If it had been simply ideograms, it might never have been decoded.

There are no known texts written in "simply ideograms". (And good reason to believe that no such text has ever existed.) Even in the earliest Sumerian and Chinese texts, symbols are used for their phonetic values.

That said, the level of "ideogrammaticality" in those texts is much higher than you might like.

Isn’t ancient Egyptian ideogrammatic?

Not entirely. Each image (hieroglyph) had also a phonetic value, sometimes so well known that most Phoenician characters were modeled after them. Basically all Western alphabets and abjads (like Arabic) stem from the Phoenician script, which is phonetic.

If each ideogram also has some phonetic value(s), does it make script phonetic? From your answer, yes Phoenician script is phonetic but there is nothing against Egyptian being ideogrammatic.

Just like if phonetic kana is based on kanji it doesn't mean those kanji, a good chunk of which is ideogrammatic, is purely phonetic.

It's an apt comparison.

Chinese writing is highly ideographic, but many hanzi contain a purely phonetic element, as a pronunciation hint.

Kana, a purely phonetic writing system, developed from hanzi, ditching the ideographic values.

It's actually the opposite: Chinese script is quite phonetic (with a lot of different base characters though) with some semantic hints. This is this way and not the other way around because of the historical formation of the main category of characters (形聲) in which new parts were added to do semantic disambiguation between graphemes used to write (quasi-)homonyms.

My impression was that 90% kanji are significantly phonetic but the remaining ideographic 10% are actually some of the most frequently used (上/下 and others)? Which is what I referred to as "a good chunk of it". But I don't know what I'm talking about:)

Early forms of many written languages had no phonetic values. We're talking history here, so 'has ever existed' seems a gross oversimplification.

It isn't possible to produce a written text in purely ideograms for the obvious reason that there are no coherent linguistic utterances in which every word has some semantic content. We can be confident that what isn't possible has never been done.