My suggestion was to leave quiet splash and append to the end:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash amdgpu.noretry=0"
This was supposed to fix it if the problem was the gpu issue.
To make sure you're drivers are up to date, though it may not be the issue, you can try:
sudo apt install ubuntu-drivers
I'll do some more searching and see if there is anything else that may be the cause.
Out of curiosity is there a specific program you are running each time the freeze occurs? Are they direct installs (apt or deb) or snap/flatpak packages (installed through the software store)?
Since the gpu didn't seem to be the issue i would remove that portion from the grub entry. Like carmar said, that iommu solution was for Ubuntu 20, it may be handled differently in 18.04.
OK, a little more reading and have a few things to try if the driver update doesn't help.
In grub, after quiet splash and removing all other entries, add:
Inside the quotes. Give it a run for a while and see if that solves it.
Do you use Chrome browser by any chance? If so, go into settings and disable Chrome gpu hardware acceleration. That fixed it for someone here: