The problem with having an astronomer is that there's no foreseeable reason why joining the Strawhats might help them fulfill their dream. With the exception of Brook everyone's dream is progressed in some way by journeying with the crew. For astronomy it doesn't seem necessary for him to join the crew unless of there's a way that his dream could progress with the journey. For example maybe he needs rare materials to build a telescope which can only be found through out the New World, similar to how Robin has to follow the trail of the poneglyphs. But so far nothing like that has been introduced. Moreover there's been nothing to indicate that Leo is an astronomer.
I see only 3 possible scenarios in which a dwarf will join
#1. Luffy pulls a Brook. "lol tiny dwarf. Become my nakama"
#2. A dwarf or dwarves must accompany the Straw Hats to the next island for whatever reason, possibly because their nakama like Princess Manshelly was sent there as a slave or something.
#3. A dwarf doesn't ask but sneaks onto the ship anyway.Overall the arc is heading towards the end game and so far none of the dwarves are expected to cross paths with Luffy before the climax. There's no time for Luffy to meet and interact with the dwarves. Not only that but none of the dwarves so far have a tragic backstory (except for possibly Manshelly having been a slave and all). The dwarves so far are also too similar to Chopper in personality (Princess Manshelly, at least according to Leo, is supposed to be bitter and selfish). I'm not necessarily ruling dwarves out but its unlikely. And if any of them join, it'd most likely be Manshelly.
I have no hard evidence that Leo is an astronomer or will even join. He just has so much potential to me as position on ship and fighter. I really think people are sleeping hard on his fighting capabilities using the Stitch-Stitch Fruit. He could pull a Scorpion from Mortal Kombat (Come Here!!!). That in itself would be gold.
This is just my speculation. Leo could be the gardener because of cultivation. Could be astronomer because of potential full name. And would be a unique individual that stands out enough among the Strawhats. But I wouldn't be surprised if nothing happens. To me it is still early though. He could still develop.
How long did it take upon introduction in the manga of each Strawhat to hear their tragic past? Dream?