Distro’s are not like picking between windows or mac, Nearly all linux distributions are based on the same linux kernel and many of the base GNU packages. The main differences between distributions are philosophical.
Some distro’s will focus on free as in speech over free as in beer meaning if something has closed source, or proprietary code they may or may not include it. You can still download and install proprietary software and drivers regardless of this initial choice.
Some distro’s will have a preferred package manager which is like their software or app store, but if you dont like the one they picked you can install a different one.
As for security, linux is as secure as you make it, its vastly more secure than Windows out of the box, and probably more secure than MacOS but we dont really know because both Apple and Microsoft dont publish their code so we cant review or audit its security. Setting up a secure linux install is dead simple and you can find dozens of guides for every distribution and edge case.
Since the main tool you want to run is Davinci Resolve it makes sense to see what distribution they test against and go with that, rather than pick an arbitrary “secure” distribution. It will be simpler to harden their preferred distro than to take a hardened distro and make their software work on it.
I suggest checking their website and going with their top suggestion.
Resize your partitions so you can use the empty space.
Then install to the new position.
Update grub.