By installing LineageOS on a galaxy s7 the old phone was turned into a self-hosted Navidrome server, with this approach you can use old smartphones like single board computers. The problem with this could be that depending on the model the battery could overheat if you leave it charging. It would be interesting to have a list of which smartphones are compatible with this approach.

I have a small ubuntu server running on a rockpi 4, I use it for everyday services like rss feeds (miniflux) and media (jellyfin). The only way I have found so far to be more resource efficient is to download movies and shows at lower resolution, this can reduce used bandwidth to a tenth. What other ways are there to save power and bandwitdth for everyday tasks?