Not a perfect solution, but as a stop-gap, you can manually trim on-demand via a command like
sudo /sbin/fstrim -av See below before running this; you want to make sure you get the right command!
systemctl cat fstrim.service will show you the fstrim service file.
Look at the bottom and see what's being executed. You'll see this after the line
ExecStart=. It will probably, for Ubuntu 18.04, be calling
With this in mind, run
sudo /sbin/fstrim -av and you'll see something like
/boot: 0 B (0 bytes) trimmed
/: 6 GiB (6384705536 bytes) trimmed
And, yes, it doesn't ask the question you're really after, which is the same question I have after seeing this question. I'll look into this myself on my laptop and post back here--it'll help us both!