Network design for a small engineering office

Hey all, I work in a small engineering office and I've been slowly grappling the IT responsibilities away from the senior engineer in the office.

Our current set up is a network of internet connected computers connected mostly via wireless to a router (1-2 computers each + phones). We have another air gapped network of 5 computers & 2 archival drives which is for the drafting stations and all our shared work. They are networked amongst each other but none are on the internet.

The problem is these isolated computers on the regular exchange USB drives with networked computers, and as they're not networked some are up to a year (or realistically more) behind in software / security updates. (These computers occasionally get disconnected from the offline network and connected to the internet for updates)

We make daily back-ups which are brought to an offsite location, the networked computers have antivirus & updates installed and generally people in the office aren't the type to open random email viruses.

Is there a better system we could be doing? I feel like we're living in the past and there's an obvious better solution. Any suggestions are welcomed.

Cheers