Pine phone Junior, message phone ?

[ryo]

A lot of the cheap ones are using KaiOS now so perhaps too googley for you.

I recently found there's a GerdaOS when reading here (Tor required to access), which runs on exactly 5 phones according to that article.
Can be found on: https://gerda.tech/ (no Tor required)

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I would really like to be able to buy a flip phone that just did VoLTE calls and SMS/MMS, didn't spy, and didn't require administering a Linux computer.

Doing VoLTE calls and SMS/MMS and doesn't spy at the same time is literally impossible.
You will need to cut out the ISP somehow to stop the spying in this case, which you need in order to make calls and send SMS to begin with.
Even if you somehow manage to do so, you'd still need to somehow get around government-owned cell towers.