So I have a TrueNAS server set up at home, and it would be cool to have access to it at all times. I currently have Syncthing set up to access and back up my most essential files on my phone and laptop, but it would be nice to be able to access all the ... legally obtained files I have stored there wherever I go.
I looked into Nextcloud, but that requires paying for a domain. So are there any other options for this?
You say that as if (normal) domains are expensive. You're gonna be paying a lot more for electricity for your NAS than a domain. If you don't need anything recognizable which you just want to use for yourself, you can even get a 1.111B class domain (000000.xyz - 999999999.xyz) which are just $1 per year. It's a much better option than a dyndns service because you can actually do whatever you need to with the domain.