I remember from my Linux Mint days that this is quite demanding on the graphics. For old hardware/low ram I use xfce/mate minimal or, if you can live without proper desktop environment, I3.
I think you’re right (first time that happened, it’s my go-to resource). Otherwise, why would there be all the arch forum links discussing booting 32bit efi?
There were discussions here regarding min ram requirements and it is indeed calamares that prevents some users to even install manjaro on <4GB machines. For me 4GB (usually minus some MB for built-in graphics, so ~3,7GB) is the minimum for a usable manjaro. I used, and setup for others, a number of systems mostly with xfce/mate, however, the no longer available Awesome edition provided by far the most responsive system on this kind of hardware. If you can live with a wm; I3 is in the same ballpark, so for a 3GB machine (and everymac is right here, my 2006 imac had 2x2GB but only 3GB available) this is what I’d try first (always download the ‘minimal’ iso).
I did some digging too with some interesting results like this script EFI-32 ISO erstellen – wiki.archlinux.de , however, this only works for arch iso. Another one is Linux DVD images (and how-to) for 32-bit EFI Macs (late 2006 models) | mattgadient.com . I also wonder if Super Grub2 Disk Beta would be able to boot an installed manjaro.
Anyway, that’s my 2 cents spent here, I wish you good luck, if you succeed post a how-to.