The self-hosted alternative to ntfy that I know of is gotify, but I have no idea if it supports file attachments, maybe worth a look.

If you go with dual booting this is what worked for me when I have done dual boot, so windows doesn’t mess with boot options:

1. Window have to be installed first or already there.
2. With a Live ISO use a tool like gparted and shrink windows main partition from the right.
3. During installation, choose custom partitioning to manage the free space

• Make a secondary /boot/efi partition so linux boot is installed there.
• Make the / root partition for linux and optional make a partition for /home.
• Finish the installation.

4. Boot into UEFI and change boot order to that secondary /boot/efi.
5. Reboot and profit.

That way grub will be running from the secondary /boot/efi, and it will detect and add windows and the primary /boot/efi

Windows only sees and care about the first /boot/efi and will only mess with that, so even if it does change something it doesn’t matter.