In practice, you need to worry about legal restrictions -- you're not allowed to transmit on common LTE bands.
Not sure how this will actually look in the future. Is the office mobile network physically separated, or do you use the public network with special SIM cards that connect to a virtually separated mobile network?
With 5G some countries have also taken into concern a more liberal spectrum policy, which could result in more mobile network operators. One idea follows is that the spectrum would be "oversold" by selling the same frequency to organizations tied to particular cities, for example.
Finally, there is the notion of virtual SIM cards. And as far as I know, alternatively you could also just hack the LTE server to accept each incoming connection (with any SIM) and use a challenge after the Internet connection is established, and then just kick out any device which doesn't send the magic packet.
I was hoping 5G had taken this use case into account, and thought to optimize for those bands, or alternatively allow for use of 60 GHz, or something where i can get a local license for usage of that band.
You can achieve network separation APNs, so there is no need for such a hack.
The question I have in mind is: Does your office advertise it's own PLMN with WiFi-like equipment, or do you outsource your office wireless network to an established operator which handles authentication and provides a virtual, separate network?
Regarding your question, I recall reading a paper which compared both cases: the office owning the whole infrastructure, and one in which it is rented/outsourced to an established MNO. While the paper suggested that both use-cases are plausible, in practice your preference is likely to vary depending on local regulation and use-case (e.g. latency requirements etc.).