How to prefer LAN range IPv6 addresses?

RFC 6724, Default Address Selection for Internet Protocol Version 6 (IPv6), regarding address selection, says that is expected:

By default, global IPv6 destinations are preferred over ULA destinations, since an arbitrary ULA is not necessarily reachable

Assuming the prefix somewhere in 2001::/16 is yours, it is local. Packets bounce off the local routers and back through the LAN, not touching the internet. If on link, direct to the destination, not even hitting a router. This can seem foreign compared to IPv4 where addresses are so scarce, few can afford to address a LAN with public addresses.

While it is possible to adjust the selection policies, do not bother. You have perfectly usable addresses that happen to be globally routable. If your ISP didn't give you a (static) prefix, ULA would be more appealing.