Hm, I see yes that would be a nice case for which to have toIQMPO ideally. However, that's also one of the cases where one might worry about whether toIQMPO would function correctly, since the long-range interactions have small tails and toIQMPO would truncate these away at some point depending on the cutoff used. But if I upgraded it the cutoff would be adjustable such that you could test it.
If you'd like to create an issue on github for this (restoring toIQMPO), mentioning your use case, then I can plan to get it working again and notify you when it's done by closing the issue. It will take at least a few weeks, though, maybe more since I have some other high-priority things too. For instance we are finalizing a version of AutoMPO that will support more than 2-site interactions.
But all that said, if it's not too hard to implement your case directly as an IQMPO then that's the best solution both for all involved. It will be more robust as you don't have to worry about whether toIQMPO modified your Hamiltonian in some complicated way.