The Mono version has very different dependencies than the dotNET version. There is no way for the built in updater to move from Mono to dotNET.
The switch would need to be made by the person that currently maintains the official port:
https://www.freshports.org/net-p2p/radarr/
The tarball found here:
https://github.com/Radarr/Radarr/releases/tag/v3.1.0.4893 is for people that want to setup on their own or would rather not deal a package. I have made an installable package for dotNET radarr that covers dependencies, service file, and ownership but it is still not for people who used the TrueNAS/FreeNAS GUI to install Radarr as TrueNAS plugins cover things like jail setup that are not covered by a simple package.
There is no real harm waiting if you are not comfortable working with a BSD shell... other than the Radarr team is moving away from Mono to dotNET and eventually you will no longer get updates for Radarr. This switch will (hopefully) happen after there is a way for people using the TrueNAS plugin system to update/migration/install the dotNET version.
The gain is...dotNET is a bit faster as it is native code. Mono 5.10 is very old, Mono 5.20 is the most stable but still very old. I have not experienced it, but others have issues with Mono 6.8.
My suggestion: wait if you are not comfortable with jail creation in FreeBSD and/or not comfortable using the FreeBSD shell to install packages.