Check if your router has an option to add custom DNS entries. If you're using OpenWRT, for example, it's already running dnsmasq, which can do split DNS relatively easily: https://blog.entek.org.uk/notes/2021/01/05/split-dns-with-dn...

If not, and you don't want to set up dnsmasq just for Nextcloud over LAN, then DNS-based adblock software like AdGuard Home would be a good option (as in, it would give you more benefit for the amount of time/effort required). With AdGuard, you just add a line under Filters -> DNS rewrites. PiHole can do this as well (it's been awhile since I've used it, but I believe there's a Local DNS settings page).

Otherwise, if you only have a small handful of devices, you could add an entry to /etc/hosts (or equivalent) on each device. Not pretty, but it works.