But I have a furrier beard and don't dev a thing.

Now the results:
After a fresh boot the systems idle @ 117 MB and 114 MB with and without the spacefm daemon running respectively. Everything else equal.
htop - daemon running - no opened spacefm instance

htop - daemon runing - one spacefm instance open (two panes with some tabs per pane)

htop - daemon not running - one spacefm instance open (same same)

The impact is not specially big.
In my personal case, maybe I'll leave it running just for the auto-open feature, but having spacefm openened and usually tabbed with the terminal and the editor doesn't make too much sense either.
Regarding session management I cannot make it work. Ignores to set a wallpaper and nothing else happens in the desktop. I remembered I've tried it in the past with the same results. Maybe Fluxbox overrides it all. I really don't know. Will install and check antix. I think I remember they made it work. Will also check the pane hovering focus there and report back.