I assume you want the cost down because with seperate apps you have higher Mendix costs. Here my thoughts:
Themes can be handled with this app: https://appstore.home.mendix.com/link/app/106033/TimeSeries/StyleSheetSwitcher
so that part is not a problem.
Creating a multi tenant app from scratch is also a doable thing. Be aware because mistakes can be made and it depends a bit how damaging this can be. Depends a bit on sensitivity of the data.
For the internal agents I would probably give them multiple accounts. So for each tenant their seperate account. Depends a bit on the work they have to do. You can make life easier for them with a autologin feature so they can switch more easy between their different accounts.
Regards,
Ronald