Larger-scale cluster have been discussed before, here’s one thread I found quickly: Scalability Question
Generally the scaling factor is devices connected. 1.4.0 has an improvement that should by far lessen the db growth per added remote device. However other overhead per device is still there. For that reason the usual advice is some kind of topology where a number of central nodes are interconnected and divide the large amount of clients (e.g. 5 central nodes each connecting to 1000 separate clients and central nodes connected to each other).
Also https://www.kastelo.net/arigi/ might be of use to you (by @calmh’s company, founder & mantainer of Syncthing).