Selfhosted

I need some advice, or at least pointers on where to go to learn more. I have been considering adding some type of backup to my network. Here is what I would like to be able to do:

1. Primarily, back up my devices, whether that be desktops, laptops, or mobile, preferably automatically, regardless of OS. I have Windows, android, and Linux in the house and would like to have the devices automatically back themselves up each night.

2. Possibly stream audio, video, and images (images to a photo frame would be cool).

3. any cool other stuff I may imagine such as more server type stuff like home automation, password vault, and anything else applicable that I haven't thought of yet. I currently have a RaspberryPi 4 running Pihole. I know I could easily add more services to this device if needed.

So I was thinking of getting a 4 bay NAS but a dedicated Synology box is going to cost me at least a grand. Would I get more flexibility AND save money by buying a N100 machine? Would this give me a machine that can be both a server AND a NAS? Can I duplicate the Synology software with FOSS? I don't know much about NAS boxes but they seem expensive for what you get.

I have no plans to expose this to the internet so security is perhaps less important to me.

Thanks in advance and please add whatever you advice or comments you may have about the best way to set this up. Again, the system is designed primarily as backup but it would be nice for it to be able to other things as well. Backing up the mobile devices is of prime concern and I'm not sure how that would be accomplished regardless of what direction I take, so any info on that would be helpful.

Thanks