Not unlockable means you can never have complete control of that device unfortunately. 

You could return the phone to whomever lied about it being oem unlocked. Pixel 6 used is around $150, 7 go for about $220. Use a service like swappa so you have some assurance you will get what you pay for this time.

I've found many unlocked, used Pixel phones on eBay. I've bought 5 of them this way for myself and my family and had no problems.

Why not buy a used Pixel?

You can buy them from Best Buy? Scamazon? Walmart?

The least demanding and least good option is probably universal android debloater, which helps you remove and disable packaged using adb. That's a bit of control on a device with a locked bootloader.

I bought a refurbished pixel 6a for 200€ from a local store with 2 years guarantee. Using it since 3-4 years and am happy.

Maybe think about a used/refurbished one?

There's nothing anywhere close to the security and privacy Graphene provides. Find something like a used 8a somewhere. Check Backmarket, Swappa too not just Amazon and ebay.

Are there even locked Pixels? Who has them.

As long as it isn't "carrier locked", you should be good to go. The thing about Pixels, is that they allow you to unlock the bootloader (and critically, for Graphene, relock it).

I paid ~$180 for my used mint-condition Pixel 6 on Swappa. You have to look up which exact model numbers are bootloader unlockable, then filter on those.

+1 for Swappa. I get all my devices there, filter by unlocked

You can find services online that people will unlock your phone for you for a fee

for some older pixel (pixel 2) there was some sketchy chinese dude that remotely bootloader unlocks the device for USD$30, tho i haven't tried that myself. most likely using some leaked engineering bootloader binary.