update: seller (which is a professional refurbisher) says that all his refurbished pixels are american/verizon and he used to flash the global bootloader via JTAG, but since the pixel 6/android 13 (pixel 6 with android < 12 can, then they patched it with some antirollover) it can't be done anymore. He says that nobody cares about bootloader unlocking and I'm literally the only one out of thousands of customers to have noticed the issue. He says that there's a way to jtag the global bootloader onto the verizon pixel 7 and allow bootloader unlocking but it will have a permanent "software tampered" warning on boot and it can't be locked anymore. Don't know how much true that is.

Update 2: I inserted an European sim card, a few hours later I got the prompt where android asks if I want to replace Google and chrome with a different default search engine or browser (mandatory by law in Europe) and most important I finally got the prompt to update to Android 15 build AP3A.241005.015 which isn't the Verizon "certified" one from 6 months ago

So at least I got this problem solved, I'm a bit sad that I can't run lineageos (seller doesn't give the super secret bootloader unlocking program that nobody ever saw and needs, but says I have to ship to him and pay an extra fee - I'm sure he's just going to swap the phone with a T-Mobile one)

If you found something, please update! I'm in the same situation