Good day! At BSC, we use *.iot.bsci.com and each product team has its own custom domain, such as divisionA.iot.bsci.com, divisionB.iot.bsci.com, etc. So, absolutely it is possible to have custom URLs for subtenancies. You’ll want a wildcard certificate assigned to your management tenant to cover the subtenancies. And Support will need to help you enter the proper DNS record for your tenancy.
The bigger question also comes up with your file repository. Are you using Azure, AWS, or the builtin repository? You may want to have that repository, if it’s not the builtin, have a custom domain as well.