From postmarketOS

Zram uses a part of physical RAM to emulate a block device and use it for swapping. In essence, in compress unused RAM content within itself, to leave more memory for the currently active one. It can also be used to host the /tmp filesystem but this is not practical.

As of pmOS 23.06, the default kernel is already built with ZRAM support, and ZRAM is enabled by default. Use swapon --show to verify. You may want to play with the default settings with zramctl utility, or alter them in /etc/conf.d/zram-init if you can offer a more suitable setup for your particular device, applications suite, and usage habits.