Do you use IIIF for this? Do you know about https://allmaps.org/ ?

No (or, not yet...) and yes, I know the Allmaps team well--it is an excellent project. There are a lot of similarities, but I would say two high-level differences are 1) OIM is built around server-side processing that immediately produces downloadable files and web services (geotiffs) while Allmaps applies a client-side transformation to non-geo IIIF tiles, and 2) OIM is designed around the creation of mosaics from many different pages to a greater extent than Allmaps is. This year I do see bringing more IIIF/Allmaps tooling into OIM though, so stay tuned!