How do I disable this popup?

What's probably happening here is your adapter is signaling some button press when you connect it that's popping up the Assistant. Only way to fix that is to get a new adapter since you can't manually disable that input from triggering the Assistant without root.

However, you can completely disable the Google Assistant from appearing at all via:

Settings > Apps > Default Apps > Digital Assistant App > Default Digital Assistant App > None

That should fully disable it in all regards and it'll never pop up again.

Screaming shut the fuck up has never worked for me, but it does help.

Try looking for "Hey Google & Voice Match" in the Google Assistant settings and disabling that.

You could have a faulty dongle or headphone cable that's causing a short and the phone interpreting that as a button press

Try diabling "Google" app

Do you hear a chime before it pops up? Maybe the "talk to assistant" gesture is being invoked accidentally/unintentionally: https://support.google.com/assistant/answer/7513985?visit_id=638282594601460963-2105321338&rd=1#zippy=

Google voice assistant cannot be turned off. How else are they supposed to listen to your conversations and create personalized ads just for your convenience?

If you got an older phone with a heaphone jack, you can skip all the setup steps after resetting to factory settings (skip wifi and data setup) and then connect to wifi, download tapdancer from apk mirror. Tapdancer runs on really old phones just fine.

Out of curiosity, are you using a bespoke device or, are you (like me) hooking a c64 tape deck with one of those car discman casettes?