I was unable to find another system. Would anyone recommend me something?
You seem to have some experience with both, do you think I am making a bad decision for a ~30 person team?
Others suggested Matrix, but I have a feeling they are implicitly assuming self-hosting. I do think Element works quite well, but I have only used it personally with matrix.org for basic chat, never for work. It does work on both Android and iOS as well as Linux, which is why I use it.
Teams is good at what it does and serves its niche well, however unless your daily matters are not well aligned with the particular framework Teams is designed for expect significant friction. It's not really the team size that matters, but rather how you structure your daily work.
A lot of the power of teams comes from integration with Active Directory, Sharepoint and Office. Sharing a presentation in a meeting that viewers can browse (e.g. to check back on something in a previous slide), calendar syncing with scheduling assistant, meetings scheduled in a team, meeting recordings and recaps, linking directly to a single page in OneNote, etc. are all quite powerful features, but most of the power is relevant if your organizational matters are structured more or less as a traditional enterprise and around AD/Office.
Inviting third parties or contractors can be quite a pain, especially if chat history is relevant. Meetings having their own chat can create information searchability issues. Integrating with third party tools is less straightforward and consequentially ecosystem of integrations is a bit of wasteland.