BTM is very promising, but it is unclear how much it scales, let alone "massively". The paper scaled it to 64 domains and it worked, but you probably want more than 64 nodes.

Since domain is actually important to its performance, you can't randomly split to 64 pieces, see Table 4 of the paper, "Domain expert ensemble outperforms random split ensemble". Performance difference is large.

So if you want to begin, I would start by researching how to scale domain split.