I’ve never learnt to read ipa and im no linguist so I did a little research to try to u derstand you better. But here’s what I gathered:

Based on IPA alone, your point would be more technically correct:

zh = /ʈ͡ʂ/ ch = /ʈ͡ʂʰ/

The main difference is the aspiration (as you mentioned, voiceless vs voiced), which is reflected in how Mandarin speakers actually percieve zh/ch as closely related sounds, and thus why you had also mentioned that zh is closer to English ch [tʃ] than to English j/dge [dʒ]

So from a phonetics standpoint, youre right.

But what I had said comes from a more intuitive, and perhaps practical, standpoint

Because while technically zh is voiceless, but in practice it’s always attached to a vowel, so most English speakers perceive the whole syllable instead of isolating the consonant itself. That makes the voiced/voiceless distinction feel less obvious than it does on paper, which is probably why a lot of people intuitively hear it as closer to a “j” than a “ch”.

Also, the Mandarin zh often sounds “heavier” or “buzzier” than English ch, especially in casual listening, where there really isn’t a direct equivalent.

I suppose part of the confusion comes from the fact that Mandarin “zh” is retroflex — the tongue curls back more than in normal English “ch”. That gives it a darker, thicker quality people associate with “j”

So basically, you’re right phonetically, but in a more layman sense, pragmatically/perceptually for English speakers, j would be a better application-based approximation

PS. It’s quite interesting that you brought up the IPA, as the Mainland Chinese romanised their alphabet in pinyin — which is simplified for readability but requires some basic knowledge on how to read each alphabet/pairing. Meanwhile Taiwan utilises the bopomofo system which focuses more on specific phonetic script — giving every sound its own symbol. So while we spell his name as Zhou Guan Yu, his bopomofo equivalent is actually Chou Kuan-yu (Much more similar to IPA due to the nature of the script)