You are missing a central coherence that ties the teams together. You are also missing one or more roles focused on integration of the work developed by each team.
Analysis and Recommendations
A "synchronisation meeting" is one of those terms that doesn't inherently mean anything. What are the teams synchronizing? What are they trying to coordinate? Why are the meetings too long, unstructured, or otherwise unproductive? Until all the teams and stakeholders perform some sort of root cause analysis or retrospective that results in actionable changes to the process, the problems will continue.
Scrum is a single-team framework. Other frameworks such as Nexus and SAFe are inherently multi-team, with the central notion that there's a coherent increment being built that requires collaboration and coordination between multiple teams. This functionality seems to be missing from your current process.
In Scrum-of-Scrums, generally the Scrum Masters (or their delegates) collaborate across teams. In Nexus, there is a dedicated Nexus Integration Team accountable for integrating the work from all the teams involved in building the increment. Other frameworks handle this in other ways, but the central idea of having a common point of integration is essential to multi-team agility.
Your next steps should be:
- Clearly identify the friction points of the current process.
- Collaborate with the other teams to reach consensus on the problems.
- Develop new roles, processes, or practices to reduce the perceived problems.
- Treat these changes as experiments, and define how you will measure the success or failure of the changes.
- Routinely inspect-and-adapt the changes until you reach a sustainable process with a predictable cadence.
Adopting a Scrum-of-Scrums, Nexus, LeSS, SAFe, or any other framework or practice won't magically fix the problems you've described. What's needed is better communication between teams, and a commitment to collaborating on building a better process through continuous improvement. If you have that, the framework you eventually adopt to overlay or replace single-team Scrum doesn't really matter. It's the communication, transparency, and organizational trust that creates success, not the implementation-specific details of how you get there.