I don't see anyone talking about the human side so I'll ask - what is the appetite for change? I can see you yourself are motivated and that's great. How do you feel the attitude is with the others there? Migrating a company that's been working analogue for decades sounds like a big change programme regardless of the tech choices you ultimately make. This sounds like process change as well as technology change and that requires using another set of skills to wrangle the people.
I would advise to pick a small area first that's causing the most pain but also very amenable to common tech most people are already familiar with and is only a small change to existing processes. Get an early visible success.
The photo management might be a good start as we all are used to these apps on our phones and the tech is mature and easy to find in FOSS.
Everyone loves Immich though it has some big warnings on its github page about its own maturity. Maybe something simpler: just file/photo synching and a shared gallery? It can always be upgraded in future. Syncthing is solid, some kind of NAS and one of the older/mature galleries running on top. Get your backup process nailed down and run a real recovery process before too many photos are at stake.
Anyway it sounds exciting and kudos to you for looking to FOSS. Good luck!