@HHeiser when support for FLAT compositions inside a contribution will be part of the spec, then this will be planned and eventually make its way into EHRbase, but I cannot give you a timeline. So if you want a solution that is vendor-agnostic, the best option is to stick to what is currently part of the specification and supported by most vendors.
(TLDR: we model a large report dynamically as a set of modular templates that are committed in distinct compositions and held together in a “compound document” composition, similar to Better’s approach in their Form Builder), and also relates to Dynamic archetype in slot based on preconditions.
This works with canonical compositions, but our developers prefer FLAT and STRUCTURED.
If your use case relies on working with FLAT composition and the problem is when committing them, maybe an intermediate solution would be to make a converter FLAT to Canonical, which you can switch off eventually when FLAT in Contributions becomes mainstream.
Disclaimer: I’m not sure if this will work for all situations, but I expect it should be possible.