Thank you for your response!
After a long time trying to reinstall TWRP, the ROM, .... i decided to try to install OxygenOS 10 back and i discovered that i had the same problem: it wouldn't let me boot into the OS.
So, i decided to download MSMDownloadTool, booted up my OP6T in EDL mode, started the factory install so it would lock my bootloader and install OOS back. And guess what, it worked so i unlocked my bootloader, installed TWRP, flashed POSP and NiksGapps and now it works! So it was not the ROM or TWRP but some random things. The only pro