>My suggestion for site admin woud be to add number of posts on each board as super easy solution or better number of posts since the last visit.
That's a good idea but very hard thing to do sadly. The nearest thing is the Discord server where each board has its own channel for post notifications, but as OP said, /kaki/'s software does not support it.

I am open for suggestions about getting more traffic to /kaki/, I value your opinions so feel free to tell me what else you think would be helpful.