I use Home Assistant with the Unifi integration that checks if a specific device is connected to my wifi, and since I and my girlfriend always have wifi on at home, this is working very good.

I know I know. Basically checks the ARP table. But in my experience phones disconnect the wifi when idle, even when told not to do so.

I send my phone a ping every few seconds to see if it is on the network[0], possibly that is enough activity to keep it from disconnecting? I use node-red personally but probably not a big difference there. Could maybe try just rigging a 'ping -n 1000000' from the command line overnight to see if that keeps the phone awake?

Although I will say it is sometimes frustratingly slow to first connect to wifi, as if it is only scanning every 30 seconds or more when not connected to a network.

[0]For lighting and ambient music to come on when I get home from work.

Wouldn't that affect battery drain?

Yes, but a ping every few second shouldn't make much difference. The problem is that phones go into power saving mode.

When using whatsapp from the website, that requires (required? Haven't used it in a while) the phone to be online, I occasionally had to wake up the phone.

I can't remember the details, but I had mobile detection reliably working using arp on my router.

I made a software to do actions when devices appear/disappear from the LAN.

https://github.com/ltworf/lapdog

It sends pings and uses the mac addresses.

Do you have iPhones? My solution is specific to the Apple ecosystem, but did indeed require some workaround with Homebridge to get it to be truly automated.

No, android.