This is just an FYI for anyone who runs into this issue.
I got the a segmentation fault error doing a clean install of 20.1 on a J3455 Fitlet2. The error appears when I hit "Accept these settings" at the very beginning of the graphical installer.
This was while using a directly attached monitor and keyboard. I did some research and found this article (https://www.neelc.org/posts/hp-t620-plus-opnsense/) which pointed me in the direction of the root cause having something to do with integrated graphics. The command in that article doesn't do anything. I then found somewhere else, I think on this forum, that I can bypass this error by installing via SSH.
I booted from the USB stick and assigned interfaces and IP to the LAN. I was able to SSH in and successfully run the installer that way.
I had been running OPNSense on this hardware for years and doing in-place upgrades without a hitch, including all the way to 20.1.8. I did a ton of experimentation without saving my good config so I just wanted to start fresh with a clean build.
Just a reminder for Fitlet2 users, update to the latest BIOS and disable all 3 SCC components in the South Bridge settings.