If the target audience is employed by a single entity (or a group of related entities, which rolls up to one entity), then enterprise deployment is the way to go. Ask the business to get a DUNS number and apply for an Enterprise Distribution certificate.
You don't need MDM for enterprise distribution. The business' certificate would be included in every app, and users would only need to approve this certificate the first time they installed an enterprise app signed by the business.
I would recommend going through the Custom App route as even though it has a higher up front cost to use in terms of time and organizational efforts, it does have additional benefits that the Enterprise program does not have. Apple is also looking to limit the number of organizations that have Enterprise accounts so getting new accounts at this time might be difficult.
The largest time saver for yourself when dealing with the Custom App route is not having to resign applications on an annual basis for your cllent. If you work with them and help them set up an Apple Business Manager account, instead of signing the application every year you will only need to update your application enough so Apple does not remove it from the Custom App Store (we see that they start requesting updates between years 3 and 5 of no updates).
Depending on how complicated your application is, the Custom App Store will have additional benefits like telling users that they have the wrong device type or a too old version of iOS instead of the application installing and turning gray because it is not compatible with the device resulting in a call to support.
Hopefully this helps!