First of all, thanks everyone for the replies and the additional reference material.
I'm trying to avoid adding active components. Yes, a TL431 is cheap, but if there's another 2021-like shortage, it's another potentially hard-to-source component to find (this board is a prototype for a small company which might become a product... and if you wonder why a clueless person like me is doing it, well, I'm mostly a digital/firmware guy helping friends)
Also, the device needs 7 channels, so I have an unused opamp to ensure is in low power mode (using two quad packages makes the most sense, compared to a quad, dual and single package).
It just so happens that the recommended way to prevent an unused opamp from using excessive power is a unity gain voltage follower with a voltage roughly of VCC/2. So if I use the voltage follower biasing from the document the @Jwillis suggested (
https://fab.cba.mit.edu/classes/863.16/section.Architecture/people/Ge/bias_opamp.pdf) with R1=R2, I can kill two birds with a stone: the unused opamp is in ideal conditions, and I get my VCC/2 bias for all 7 opamps.
Even if the voltage bias were to be slightly off VCC/2, it won't matter as much because all 7 channels would be identical and I'll have to calibrate each channel in software anyway (but thanks for pointing that out)
Something like the circuit below with R1=R2=4.7MOhm for the reference voltage (including the bypass capacitors) and Vbias connected to the positive input of all 7 channel opamps should get me what I need