This sounds easy, but is really hard to implement, because ignores are applied at scan time, not at “send stuff to the other device” time.
Furthermore, information about all items that are not globally ignored need to be sent to the other side, because items are numbered as we detect changes during the scan, so you can’t just skip item 10 because its per-device ignored, as we have delta indexes.
So we when devices send information to each other, they say “I’ve seen everything upto item number X”, and we send only items higher than X, now lets say you skipped item 10, went past to item 20, and decide per-device unignore item 10, we have no way of discovering that that was never sent.
What you suggest needs pretty much per-device index/scanning, which would be balooning the database into n^2 territory.
I think you either need to break down the problem, have more granular folders that you share selectively, or use something like arigi, which is commercial software for centralised syncthing management.
Also, perhaps you don’t need bidirectional continous sync and there is a better software product for your problem.