Hi, I have Solus on my iMac Late 2009 i5 32GB RAM who is running the 2010 iMac Radeon HD 5750 after his Radeon HD 4670 GPU broke (a very common failure). I have no problems on Solus 4 latest but I was unnable to install using the Solus 4 ISO, stuck the same way like you. Solus 3.999 ISO doesn’t work either. You’ll need a Solus 3 ISO and make the bootable USB with Etcher on macOS (I did use Etcher v. 1.3.1) You’ll be able to install Solus 3 but not to upgrade right away. It's a too old ISO to find the package repositories. After install and boot on Solus 3 you need to do this:
Open the Terminal and type
sudo eopkg dr Solus
and hit Return/Intro key
then type
sudo eopkg rr Solus
and hit Return/Intro key
and finally type
sudo eopkg ar Solus https://mirrors.rit.edu/solus/packages/shannon/eopkg-index.xml.xz
and hit Return/Intro key
After these three commands on the Terminal type
sudo eopkg upgrade -y
and hit Return/Intro key
You’ll see that Solus begins to upgrade. Let it finish. If you get some errors (I think those are shown on red text) just re-try until it finishes without errors.