XPS 13 9310, Ubuntu, deep sleep missing

My computer drains a lot of battery when in sleep mode.

I've tried following the steps outlined in this bug report, tried the diagnosis layed out in this question and disabled the "sign of life" options in the BIOS as described here.

I confirmed my system sleeps in the `s2idle` state. Which I suspect is the problem.

$ sudo journalctl | grep "PM: suspend" | tail -2
May 13 18:41:00 mex kernel: PM: suspend entry (s2idle)
May 13 20:52:36 mex kernel: PM: suspend exit

However I have 2 issues when trying to follow the above guides: Firstly my `/sys/power/mem_sleep` file does not contain a `deep` option:

sudo cat /sys/power/mem_sleep
[s2idle]

And secondly I can't seem to edit the file to add a `deep` option:

$ echo deep | sudo tee /sys/power/mem_sleep
deep
tee: /sys/power/mem_sleep: Invalid argument
$ sudo sh -c 'echo deep > /sys/power/mem_sleep'
sh: 1: echo: echo: I/O error

Not being able to close the lid and have the laptop suspend is very annoying. Any help on this would be greatly appreciated.

OS: Ubuntu 20.04, 5.8.0-26-generic / Hardware: Dell XPS 13 9310