Indeed. If your Nextcloud instance can/shall be available through Tailscale only, you can just choose an appropriate hostname within your Tailscale network and use it for DNS at the clients. DDNS is not required at all in that case, as Tailscale manages everything.
How to do that would be actually nice to have in our docs: VPN Software Options - DietPi.com Docs
Might differ from platform to platform, but at least for the most important ones, or for Linux systems, we might want to add a docs tab. Are you open to write down the needed steps, probably even contributing it to the code directly? DietPi-Docs/docs/software/vpn.md at 259255c5c4f86d27c5808b72a1242403e8c39b53 · MichaIng/DietPi-Docs · GitHub