My answer is based on the assumption that the time period is medieval or earlier. You define "close relations" as meeting every 5 years. This implies there is no electronic communication or phones, etc. And that transportation is slow. Other things you say (and mostly what you don't say) point to this as well.
Instead of looking to which modern nations (spoiler alert: none) have been at peace for 500 years, go back in time to find small nations that got along for prolonged periods. Remember too that what was called "kingdoms" way back then, were often what we would call cities now, with perhaps a day's travel between them. Not always of course, and there were early empires, but often.
We see some candidates in West Africa. While these kingdoms tended to engage in wars and the like in the 11th century on, it appears that things were a lot more stable in the centuries before then. With some of the kingdoms existing since 800 CE.
Aksum in East Africa was pretty stable from 1-600 CE. Ghana in West Africa (also discussed in the above link) began in 300-450 CE and lasted many centuries. And many others. Presumably, there were multiple smaller entities one could call kingdoms that traded with each other reasonably peaceably.
In what is now Peru, the Norte Chico civilization flourished between the 30th and 18th centuries BCE. Several others existed for long periods. We can't conclude that this means they were at peace with their neighbors (or with smaller entities within their borders), but it implies there was stability of some sort.
Then there was the Iroquois League of Nations. Five nations working together since perhaps as early as 1142 CE. In the early 18th century, a 6th nation joined them.
The Iroquois Confederacy is believed to have been founded by the
Peacemaker in 1142, bringing together five distinct nations in the
southern Great Lakes area into "The Great League of Peace". Each
nation within this Iroquoian confederacy had a distinct language,
territory, and function in the League. Iroquois influence at the peak
of its power extended into present-day Canada, westward along the
Great Lakes and down both sides of the Allegheny mountains into
present-day Virginia and Kentucky and into the Ohio Valley. The
League is governed by a Grand Council, an assembly of fifty chiefs or
sachems, each representing one of the clans of one of the nations.
We can pick apart any one of these examples. I'm sure none of them lived in perfect harmony with their neighbors. But, yes, what you're asking for is possible. In the absence of a more powerful kingdom whose goal is to conquer, smaller states can live pretty harmoniously if they choose.