Following the Wiki I put “discard” entries into my fstab(5) for swap (but not / as it’s suggested to use fstrim instead, and I had noatime for /boot and relatime for / already) and changed the scheduler. What wasn’t written there was to set vm.swappiness=0 in sysctl.d/local.conf *shrug* but it helps.

What they also didn't mention:

mount -t tmpfs swap /var/cache/apt/archives

And in sid, which my dayjob-laptop is running, APT _finally_ creates the missing “partial/” subdirectory itself. Thanks!

I had the filesystems already created, so I changed the ext options with tune2fs:

tune2fs -E stride=1024,stripe_width=1024 /dev/sda2 # 1 KiB blocks: /boot
tune2fs -E stride=256,stripe_width=256 /dev/sda4   # 4 KiB blocks: /

I wonder how well that works. I also did _not_ manage to find out my device’s flash block size (search engine fodder: erase block size), so I assumed 1 MiB (also used for partition alignment already) as worst-case scenario. Input welcome on how to find *that* out.