Small addition in case it's useful: on the E5800 with a PPS charger the charging runs through the SGM41600 switched-cap charger rather than the SGM41542S buck (VBUS 8792 mV vs VBAT 4288 mV, ibus about half of ibat).
Setting charge_en = 0 on the SGM41600, plus the buck's vreg below the cell voltage so it doesn't take over, gives 0 mA into the cell with the router still running from the adapter.
I put it in a script: GitHub - ChiliApple/mudi7-battery-limit: Charge limit for GL.iNet Mudi 7 (GL-E5800) - stop charging at a configurable SoC while running on USB-C power · GitHub — glbattlimit on 80 / off, releases itself on unplug, nothing written to flash.
One thing to watch: the GUI percentage isn't the fuel gauge value. On mine GUI ≈ 1.39 × gauge − 19, so a gauge limit of 71 reads as ~80 on screen. The script converts, but I only have four data points so far.
Happy to be corrected if I've misread something.