O/S
/tmp as tmpfs - https://wiki.archlinux.org/title/tmpfs
logs that don't go on forever - https://github.com/azlux/log2ram
systemd logs that don't go on forever /etc/systemd/journald.conf:
SystemMaxUse=50M
Faster, more responsive kernel (if stuck on an LTS for everything else) : https://liquorix.net/#install
Ubuntu Specifics
Get rid of 'advantage' and ESM Spam https://vi-server.org/pub/fake-ubuntu-advantage-tools.deb (via https://askubuntu.com/a/1438510/1681137)
zswap - add zswap.enabled=1 to GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub
zram (faster, in memory swap to be used in preference to real swap) - https://askubuntu.com/questions/174579/how-do-i-use-zram
Much smaller kernel images in /boot - set COMPRESS=xz in /etc/initramfs-tools/initramfs.conf (per https://askubuntu.com/a/1429218/1681137)
Reduce Snap disk space usage by >30% : sudo snap set system refresh.retain=2 (https://superuser.com/questions/1310825/how-to-remove-old-version-of-installed-snaps)
Apps
Work around Snap's not being able to see /tmp : https://askubuntu.com/questions/1263843/how-to-allow-snap-applications-to-access-tmp-folder
FireFox
MultiRow tabs : https://github.com/onemen/TabMixPlus#installation
Tabs that look like tabs : https://www.userchrome.org/firefox-89-styling-proton-ui.html#tabstyler
LibreOffice
Add updated LibreOffice PPA : https://launchpad.net/~libreoffice/+archive/ubuntu/ppa
GIMP
As a flatpack to get the good plugins working : https://github.com/bootchk/resynthesizer/wiki/Install-Resynthesizer#flatpak