Until we have topical relays, I think the simplest approach is for each client to categorise the typology (what nostr:nprofile1qqswuyd9ml6qcxd92h6pleptfrcqucvvjy39vg4wx7mv9wm8kakyujgpypmhxue69uhkx6r0wf6hxtndd94k2erfd3nk2u3wvdhk6w35xs6z7qgwwaehxw309ahx7uewd3hkctcpypmhxue69uhkummnw3ezuetfde6kuer6wasku7nfvuh8xurpvdjj7a0nq40 is doing with Gossip) and randomizes the assignment, maybe trying to match the user with the nearest geographic relays, to incentive a first aggregation by culture and language.

A variant may exploit an invitation system, such as that of Coracle, and assign the new user the same relays as the one who invited in order to maximise the visibility of both parties.

Finally, when topical relays will bootstrap, probably thanks to NIP-29 groups, a simple "pick your interests" wizard could improve the assignment.

Actually, here I see more engineering than UI, at least for the onboarding phase.
Instead for more advanced users, I would try to surface relays by country, social graph usage and topics.