I do the same thing - multiple properties, multiple owners - and yes, I know the difficulties of notifying owners!
I list properties on AirBnB / HomeAway / TripAdvisor and Booking.com. I also take bookings directly. In due course I'll take bookings via my website too.
As I have 20 years experience in website development, I have built my own website system that downloads the calendar data regularly and stores it in a database. My website displays that data for me so I can see all bookings, changeovers etc etc. I can edit every booking to store additional data for each booking - which housekeeper prepared the property and what review score it got - whether guests have a dog - etc etc etc.
This is all for my own purposes at the moment. Soon, I hope to have it generating statements / invoices to the property owners. And later, property owners will be able to log in and see all their bookings and payouts. And ideally, it will email owners when they get bookings.
It's a massively complicated system and will do a lot more in the future. I have the skills and knowledge to build it - but I don't have time at the moment.
So for now, I'll just message owners on Messenger .... and yes, they get excited and they want to chat and I have even less time to build my system!
There are several Channel Manager software solutions available. I've seen one that apparently allows us to manage properties for the owners and give the owners logins and notifications, but can't remember which one - I didn't look into it as I know they won't do some of the things I want, hence building my own.