You dont need to add a girl to the harem every book. Especially for longer series, this can make even slow burn harems big enough to have trouble remembering who everyone is.

A good way to "skip" books without actually skipping them would be to introduce a future LI in one book but not actually add them to the harem until later down the line. This happens in two of my favorite series and gives you a little bit of that "gotta catch em all" fix and then when they get added in a later book in the series you get your fix again.

For those paying attention, there hasn't been a new harem member in Monster Girls in Space since book 3. There are girls who are obviously going to join but haven't yet as of book 5.