You might try Tailscale or Wire Guard. Either can be used to create a mesh VPN that can include any device you want. Connect your devices to the VPN then you just access it like it is on your local network. Of the two I use Tailscale. Dead simple to setup on pretty much any device.
I looked into Nextcloud, but that requires paying for a domain
Depending on what installation method you choose to go with, you don’t need a domain. It’s just very much helpful to have one. Especially if you decide to have it public facing. Plus domains are cheap. A bigger issue for us self hosters is dealing with dynamic IPs. Most of the time you can buy a static IP from your ISP, but if that is not an option, most domain providers provide a way to deal with variable IP addresses.
And yes, Tailscale does ignore dynamic IP addresses. I think Wire Guard does as well as Tailscale is built on Wire Guard.