It is possible to optimize for the least ram usage - but unused ram is a waste. I have released Manjaro Openbox ISOs which idles at 100MB. But it’s a long time since I cared about how-low-can-you-go - it really doesn’t matter today.
So of course for low memory systems it may be an issue but as Manjaro don’t support 32-bit and the wast majority are systems with 4G then it becomes an academic discussion.
I have built the LXDE/LXQt/Openbox based community ISO for years and although they are not listed on the web I still do - and a few other members build some spins too check the #contributions:spins category.
The latest LXQt build is idling around 500mb using a default Manjaro setup with firewalld enabled.