My small army of old android devices are plugged into a USB 'charging station' (multiple USB charging ports off a single plastic device) which in plugged into a smart switch. The smart switch turns on for a couple of hours overnight, to keep the phones charged at least minimally.

Most of the devices also have a custom ROM and are rooted, and using the ACCA app I restrict charging to a maximum of 80% battery capacity.

I had a Samsung Note 5 (released in 2015) that only went spicy maybe 6 months ago. I have a Samsung S9 and a Nokia 6.1 that are both still going strong with fairly recent versions of LineageOS. Both are 2018 phones, so around 6.5 years old (old for phones, but shouldn't be 'electronically' old).

> Most of the devices also have a custom ROM and are rooted, and using the ACCA app I restrict charging to a maximum of 80% battery capacity.

I used this method too but then LineageOS merged the functionality in a few years ago and it works perfect.

I have a Nokia 6.1 as well but my oldest continuous use device is a Oneplus 8T used to provide hotspot to a location 24/7/365.