Your remarks make it sound like there’s no real reason for the number 70 exactly, so I’d suggest not going with a fixed cutoff point, but rather evalute a solution using low and high watermarks. Start creating new channels at a low watermark, but also make it so only the high watermark makes that number of users be a problem in a channel. That way if people are added to a channel after the low watermark is reached isn’t directly a problem. You can tune those numbers against expected latency of coordination between nodes as well as expected arrival rates of users. This makes even more sense if you seem to be moving people around anyways.
The part about people potentially landing in empty channels cannot really be avoided though. There will always be the case of all existing channels being considered full and the new person being put into a new channel. It’s however a matter of how likely you make that case. E.g. if your balancer does its job how likely is it for that case to happen.
I have no idea about scaling persence, but I’m asking because both likely affect performance in distinct ways. There’s no single knob to dialing performance. I’d also argue that performance work is best evaluated by benchmarking your solution besides reading what others have to say.
