As to whether installing an AOSP-derived Android distribution is easy to install or not mostly depends on whether the device is locked in some way or other. If it is you'll need to unlock it which - again depending on the device - can be as easy as flipping a switch or as hard as trying to get some code from a vendor which doesn't really want to give that code or which might not even exist any more. Installing a distribution is no harder than installing Debian, keeping it up to date is easy as it supports OTA updates. Once you're over the first hurdle - unlocking - it should be easy enough.

As an alternative you could root the device and disable (or remove) the bloat in the stock distribution, this often goes a long way towards getting a more usable device. This is what I did on those few devices which I've used on the stock distribution - remove as much bloat as possible, remove all social media cruft, etc. This can lead to surprising results like the battery lasting twice as long and with that giving new life to a tired old device.