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.
Determined by Google, GrapheneOS will probably provide extended support like they did for the Pixel 4.
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.
As far as I know you can't, but what I did was to connect the drive to a PC (Windows or Linux) and make a SMB share of it. There is an app on F-Droid that can connect to SMB shares.
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 use this all the time, also nice for text share.
And recently Llama3 and Mixtral!
It's better than nothing, surely better than proprietary software.