Multiple boards viewed in one board w/ timelines

We currently have 15 client boards we want to be able to see all timelines in one view (e.g. a gantt view). Currently we’re creating a single board which we then add groups/items for each client (like an overview from each client board) so we can also have a group for employee holidays. With this we can view project timelines and holidays. The problem is we’re essentially doing double entry - we enter data into the Scheduling board AND into each client board as well. I’d like to find a way to not have to have a Scheduling board. I had hoped “My Work” would do this but it just doesn’t have enough functionality (yet).
If I need to continue with a Scheduling board, I’d like to find a way to mirror the other board information into it instead of needing to continually update it with duplicate information.
I’ve tried to use a dashboard with timelines but we are limited to 5 boards. I read a thread about a Master Board - but it uses automation to update the master and we’re already using too many automations for our account to add more in. Surely I can do this easily with mirroring information into a single board?