I've used Invoice Ninja for awhile. Works great. Self hosted but not foss.
hydrogen
"Cloud" storage is indeed more expensive. But depending where you live, you count the electricity cost in, and you use the storage 'only' for backups. Maybe it makes more sense to pay for remote storage in a datacenter. Check out Hetzner Storage Box, it's what I use.
If it's still too expensive, maybe ask a friend or family member (maybe someone that uses your media) to setup a nas at their home for backup purpose. (I use this for my media)
Make sure you encrypt your backups if you use a remote location for your backups.
You have to decide what's valuable for you. For me my media is, I can just download everything again, but the time I put in to have every movie the correct subtitles without ads, the correct posters, metadata etc. I value my time, I don't want to do it again if I loose everything.
You have to flash an extra file if you need the proprietary Google Play Services, at default LineageOS is just stock android without it. MicroG is is indeed an open source reimplementation of GPS, so it spoofs your device as it has the real GPS installed.
Also it uses other (local) database of network locations (UnifiedNlp) so when using a maps app it will find your location faster and more accurate than only relying on gps (the positioning system, not Google Play Service) ;)
In my opinion I feel like /e/OS is too bloated and I don't like the iOS inspired theme. If you prefer stock android you should choose LineageOS. BTW there is also a fork of LineageOS called LineageOS for microG, should you need Play Services.
Take out the hard drive out of your laptop and put the drive for the server in it, install Debian using the built in monitor and keyboard of your laptop.
I've a Dreame L10s Ultra flashed with it, very happy with my purchase. A bit scary to install everything but if you read the instructions before doing it you should be fine :)
I use SplittyPie allot with my friends if we go on a trip/do something together. Works really well. You can install it as a PWA and it's FOSS ;)
RSS Guard seems to have support for Google Reader API, so FreshRSS is also supported. It's also a Qt application. You can also find other clients on the 3rd party clients page of FreshRSS
It's better than nothing, surely better than proprietary software.