depends.. a lot of the time factor in implenenting it is just the risk factor (IE, integration testing and or finding that some solutions may just not work as advertised). So by risk I mean that, a lot of time may be spent going in a direction that wasn't going to work or could incur unknown integration debt.

Habla

A Guide for Relay-Based Group Implementations - hodlbod

A guide for people who want to build clients for "relays-as-groups" that are compatible with Flotilla and Chachi.

GitHub

frith/go.mod at master · coracle-social/frith

A relay which enforces authentication based on custom policy - coracle-social/frith

is the only relay implementation that already says it supports, but is on a now depricated version of khatru library.