Our clients often need to download formidable spilling ranges from their ERP systems in a single custom function call.
The minimal repro repository mentioned in this bug: https://github.com/OfficeDev/office-js/issues/2786 emulates the GI() function in Velixo add-in.
It's one of our most popular functions in our legacy XLL product and a big selling point for our company. It routinely returns dozens of thousands of rows which the customer pivots, sorts, filters, and aggregates in Excel.
It needs to work in exactly the same way across the legacy/new office.js based product, otherwise we simply can't promote migration to the new cross platform solution.
There is no workaround to the problem which makes it so urgent and painful: we need backwards compatibility with our old product so it needs to remain a single custom function call. As per MSFT in the GitHub issue, there is no addin <-> host request batching within function calls, only across multiple different function calls.
IMO the best solution to this would be to abstract away any payload size limit for CF return values from the add-in developers — and automatically batch those requests in chunks.