What filling up means is that you have 16gb of ram, and if your system tries to use it all, it will freeze, crash and require hard rebooting. I have more than double your ram and have hit oom (out of memory) which is a kernel error. Is your do some research around out of memory and how to mitigate and prevent it in Linux, you’ll find multiple solutions and how to check if it is.

Could be oom? How much ram do you have and is it filling up?

Honestly dd is perfect. Like you say, shrink the partition down first, DD to the new disc, swap discs. That’s it.