I'm polyamorous myself, with a girlfriend of about 18 months and another of nearly a year. Both my relationships are stable and very fulfilling, and also relaxed and laid-back. It takes more communication to have it work but for me I can't even imagine living any other way, polyamory feels right for me and me and my partners are happier than we've ever been.
Granted, my relationships aren't a case of opening an existing partnership, but rather I talked about the fact that I'm polyamorous to each partner very early on before we even considered a relationship. Most drama I've seen in polyamory comes from one partner in a monogamous pair wanting "more" and so the decision is pretty one sided, and neither is willing to really put in the work and communication that healthy polyamory requires. Every polyamorous person I know that started their relationships as polyamorous is healthy and happy in their partnerships.
I'm really happy it's working out for you! I'm not saying that all existing partnerships opening to polyamory are doomed to fail or are inferior in any way, just that from what I've seen they're much harder to pull off, possibly because people tend to open relationships for the wrong reasons. But when it does work out and everyone involved is entirely on board and is willing to put in the work, they can absolutely be beautiful and healthy relationships.