Just going to post this workaround here since this is one of the top Google results / a lot of links point here when you're looking for mono upgrade solutions and manually compiling on each of your jails or anything along those lines can be quite tedious.
This thread has the secondary packages needed in order to install mono 6.8, as well as someone posted the mono-6.8.0.105 patch pre-built to GitHub for ease of download.
The excerpt steps are below:

I don't mean to spam/bump, so I apologize if this is interpreted that way.
I am just asking if anyone has plans to accelerate the merging of this review (and the necessary changes), since it has been around for quite some time (the bugs.freebsd.org port has been since 05-24-2019 and this since 01-21-2020 while the version still active on FreeBSD is incredibly dated (~5.10.1.57) and actually won't work for a few of my services. I would manually install the patched port files for my systems, but it's not practical to maintain for > 1 jail (let alone 8+ jails). 😛
If there are no active plans (which is okay, of course), I will start making plans to move my systems away from using FreeBSD as the host to a more containerized system (i.e. Docker/VM instead of my current Jails), and just want to know if that is going to be necessary! 🙂