I once had a client who wanted to make a web app out of a spreadsheet that relied on Excel's optimization functionality. I wonder if that would work? They might still be interested in it.
I can't say for sure, but likely not. It sounds like your client may have been relying on one of the optimization add-ons that are automatically installed with Excel[1][2], rather than actual Excel features. The company that makes those add-ins has developed new versions that work with Excel Online, but add-ins in Excel Online execute in the local browser context. So I don't think they're loaded/usable when you create a headless workbook session via the Graph API (although I've never actually tried to do that, so could be wrong).
That said, Frontline Systems (the company that makes those Excel add-ins) does have a web API[3]. The optimization models there are a superset of the capabilities in the Excel add-ins, so your client's Excel optimization model could likely be ported over to that pretty easily.
[1] https://support.microsoft.com/en-us/office/use-the-analysis-...
[2] https://support.microsoft.com/en-us/office/define-and-solve-...
The problem is handling Excel instances.