I have the same use-case and there seems to be a solution posted in the LlamaIndex forums. https://github.com/run-llama/llama_index/discussions/8432 Pattern is:

• Combine all indexes using GPTListIndex
• Create a graph (ComposableGraph) from this combined index and save it to disk
• Create an index while loading this graph from the disk

I am in the process of trying this out but this is the suggested solution.