"We also looked into tools for XSD to OWL transformation. Most tools make certain assumptions on XSD modelling approaches and if yours is modelled differently then a lot of "post-processing" is necessary.

One free option you could try is ontmalizer https://github.com/srdc/ontmalizer. Then there is the commercial topbraid composer https://www.topquadrant.com/products/topbraid-composer/ which can also do this conversion. Last time I checked they had a trial version."