and how to increase it and still have a functional phone
For starters, check whether you’re connected to 2.4GHz or 5GHz WiFi network. The WiFi module currently used on the Librem 5 is known to be power hungry on 5GHz networks. If possible, switching to 2.4GHz may make a noticeable difference.
Another thing - it’s not enabled by default because it causes incompatibilities on some WiFi networks, but there’s a power saving mode that you can try enabling and see how that goes:
sudo iw dev wlan0 set power_save on
If that works well enough for you, then you can make it the default in NetworkManager the same way you can do it on PCs (it can also be set per-network IIRC).
Other than that, powertop is a great tool to check which processes are waking the phone up when it’s supposedly idle, so you can use it to catch whether some process goes rogue on the battery in the background.