I would recommend using caution with this, but you could list all packages and look up what the package does; and if not used, then remove it (I also recommend a clone before you start, plus a clone when you are at a stage you like the result). Check the output of:
dpkg -l
# Or with a pager:
dpkg -l | less
As an example, I see “thunderbird
” on one, and could “sudo apt remove thunderbird
”.
Perhaps there is some sort of compression tool you won’t use, but that could turn around and bite you back since other software may only show they depend on it after a long period of testing.