I'd add that another difference is the 7a has Tensor G2, while 8a has G3 and 9a has G4, so it also depends on your needs.

I'd narrow it to 8a vs 9a and then just compare cameras, RAM (same iirc), battery and whatever is most important to you.

I've not been shopping for pixels so I can't say much about the 9 series, but at least that narrows it down.

If I cared about cameras: I would check whether the lower megapixels on the 9a that's supposedly still better than 8a's due to AI enhancements and software tricks still applies on a googled phone.

Since I care about security and use GrapheneOS: I would personally see if there are any big differences between the Tensor G3 and G4 chips in terms of hardware and security features they enable.