Hello there,
adding someone this late in development would just slow things even more, they would need to learn how the project is programmed, how the systems work, as well as it would be adding another person to the list of a free project. The size of the team is in a good spot to what Kal wants to accomplish. The updates are normally released in between a couple of months (3 or so) and most of the times what causes the delays is the simplest thing... Dev wants to keep adding stuff. For example, 1.8 wasn't supposed to be a big update, but the number of reworks that happened caused it to grow more than expected; same as 1.8A, normally lettered updates are just bugfixes, but this one will also contain some extra content just like 1.5B did in the past.