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.
One major downside is space, but if you have like 1 or more terabyte, you should be good. In the past, Windows updates have been known to eat the GRUB boot loader but I'm unsure of the current state of that issue.
I have like north of 2 Terabytes of space. Dont think thats gonna be an issue. And windows and linux will be both on different SSDs
Plus afaik there are no more new updates for Windows 10 so that shouldnt be a problem.
Also, if you are planning on gaming on LInux side, grab this. It's a manager for the Glorious Eggroll fork of Proton and it makes trying to get a wonky game to run so much easier. I started playing Warframe and could only get it to run with a really old GE-Proton release because of this.
Yeah so if you are keeping Windows for, like, gaming and stuff I bet you'll be fine. I think outside of maintaining your Linux side updates, the only real annoyance you'll have is switching OSes when you want to.
I dual-booted back in 2015 I think but finally made the full plunge to Linux after some hardware upgrades nuked my Windows side. But again, if this is a new build, I don't think that will be an issue. And last I played with it, Win10 is better at hardware upgrades now.
Long and short, you'll probably be fine.