Buy one used to not financially support them. GrapheneOS only supports Pixel phones as they are the only ones which meet the security requirements.

https://grapheneos.org/faq#supported-devices

I bought my Pixel 9 refurbished (as new).

Refurbished is a thing. So is 

A while ago they announced they were planning a collaboration with a big phone brand

Nothing else meets the secuirty requirements and they don't want to comprimise.

https://grapheneos.org/faq#future-devices

The level of security, of privacy and of granular control of those does not come even close to what GrapheneOS brings to the table. Additionally Iodè and /e/OS have some noise going on with not being private at all. And all of them, except LineageOS (some times) have long delays with security patches.

/e/OS uses OpenAI for voice-to-text. Iodé has closed source components (trust me bro?). What do they claim? "some protection is better than none". In security that is ridiculous.

Then there is choosing between Microg vs actual sandboxed Google Play Services (which you don't need to have at all on GrapheneOS if you don't want to).

/e/OS signs their OS with Google test keys instead of their own keys.

Unconfirmed, but concerning is the claims of ties of those 2 (Iodé and /e/OS) with the French government, not unlike Chinese companies with the CCP.

Choosing LineageOS, or even CalyxOS are options I would consider if I wasn't willing or able to get a Pixel device, but Iodé and /e/OS are absolute 'no' in my book. Too many holes, too pragmatic (and I'm trying to be nice using that word) for my taste.