Whatever the niche topic is, when there are multiple communities that maybe be broad enough to contain your audience, there’s probably a lot of overlap in subscribers. So yes do crosspost, and do it immediately, but only cross any given post to 2 or 3 other communities. Then it won’t feel like large spam to people subscribed to all of them. I prefer to scroll by a set of 2-3 crossed duplicates, one right after another in the feed (because they were all posted at the same time) than to have one post seem to repeat with some time separation.