Ask Experienced Devs

Would Syncthing work?

You tell it a folder and it will sync the contents of the folder between machines.

Nextcloud is pretty easy to setup. You can use it just like Dropbox/OneDrive. Just drag files you want to sync into either the web interface or a mounted directory.

With your minimal requirements, a Raspberry rumning stock Rasbian or Armbian, and SSH/SCP commands (and an Android/iPhone SSH/SCP client) could do the trick.

The disadvantages to this approach is you'll have to learn the commands you need for every use case. The advantage to this approach is you'll have to learn the commands you need for every use case, and we would learn a lot of useful stuff.

Source: I used a Pi for this need, before I bought a fancy Network Attached Storage (NAS) device.

Edit: Further pro/con:

• It'll work with almost everything. SSH and SCP are very old and stable and there's tons of apps.
• Too many options. I had to pick my clients a bit at random, and research and switch to find what I wanted. Even my first random choices did work fine though.
• None of the SCP apps for phones have particularly nice interfaces, in my opinion. This is a minimalist solution, and it feels like one.
• Nothing proprietary - no one is going to Google-kill this solution due to lack of profits.
• All the software you need for this is free.
• This solution can be extended in tons of nice little ways later. I.e. something like AWS CLI for trivial off-site back-up to a secured S3 bucket.
• A well chosen NAS gives all of this functionality, and much more, but for a lot more money.

You probably wanna take a look at "Nextcloud".

Truenas or Nextcloud or syncthing.

Sibling comments haven't addressed the bit about how you want to protect against fire. A NAS or Pi in your home won't protect against natural catastrophe. You need something remote.

A device that runs in someone else's home (like a family member) with a remote connection could work, but it's likely more hassle to setup than what you'd want. You'd have to punch through firewalls and such. Tailscale offers services that apparently make this easy, however. I haven't used them and couldn't say.

Since you seem to be working with small files, I think you should look into a Virtual Private Server (VPS). You can get an instance for $5-10 / month with someone like Digital Ocean or Linode or others. You would then need to enable FTP and get an FTP client. Depending on your preferred platform, this might be pretty easy or not so easy. On Mac, I'd recommend Transmit, by Panic Software. It's awesome. You can also transfer files via SSH and SCP if you're comfortable on the command line and not use an FTP client.

Encrypt the data and upload it to 0x0.st :)