I plan to do a pc upgrade very soon. Alongside that I plan to start with Mint or Fedora. Is there any real or big downsides to dual booting ? Aside from the harddrive space lost ?
If I like or really like my time with Mint I would probably switch permanently but I felt like I wanted to test it for a couple of months before making a complete switch.
Im a big 
so that impacts my consideration. Dont really play much multiplayer shooters so I dont have a problem with kernel anticheat games not working.
They shouldn't fight as long as you're using UEFI to boot, which has been required by Microsoft at least since Windows 8. You should not be installing any boot loaders via the MBR method in 2025.
Linux distro installers typically support booting via both BIOS and UEFI (from the same disk image), and will usually install using the mechanism that they were booted with. BIOS boot compatibility should be disabled in the UEFI settings first to ensure the installer and resulting OS boot with UEFI.
I dual-booted for a long time (stopped recently). Since Windows XP. The main downsides are partiton clutter, needing to do updates on two OSes, needing to do peculiar configurations to keep the system clock correct, preventing hibernation, and limiting your filesystem choices to ensure cross-compatibility. Otherwise, it can be useful, but I found myself booting Windows once every 3 months.