Which Ubuntu?

[SDA]

The OP could consider a slimmed down OS like AntiX IMHO. I'm running it on a 
12 year old laptop with 4 Gb of ram. Modern Gnome or pretty much other DE 
won't run good on this - yes I've tried.
So with AntiX i've presently have open a terminal and a heavy browser 
(Firefox-ESR) both using about 980 Mb of ram! This distro also uses slimsky 
as the GUI rather than XOrg.
Other considerations within the Ubuntu ecoystem might be a tiling window 
manager, but it sounds like the OP want's a GUI window manager. AntiX 
defaults to Icewm with the zzzFM (file manager) and Rox panel. It also 
doesn't run any systemd (I'm not anti systemd) which probably helps slim 
things down too. YMMV

AntiX: https://antixlinux.com/