It sounds like we’re both in the exact same automation-development industry with different clientele. 
There’s always a million ways to solve everything, so I can’t say this is the right answer. I can only offer what immediately came to mind.
- If your source of truth is external, then you want to make sure any changes made there will trigger changes in HubSpot’s records. Event listeners can trigger SDK calls over to HubSpot to keep your data in sync. You could also write up something that fires off after a period of time to double-check.
- That’ll be up to you. It may be overboard to whip up a PWA or SPA if it’s just team-facing, so perhaps some server side rendered application to interact with your data?
Hope that helps!