There isn’t a /40 vs /50 thing like there was in RO. 15 class is where the advancement becomes available and you can’t go any higher, so swapping at 15 is just fine.
You do another class change once you reach class 15 of your current class, much like you had job 10 for Novice, then 40-50 for Acolyte, and then 50 for Priest. So atm you are Cleric > Priest. If next circle you choose Priest again you will be Cleric > Priest 2, if you choose Bokor you will be Cleric > Priest > Bokor.
I don’t recommend Cleric 2, it gives almost nothing compared to the first circle. Paladin is a rank 4 class, so if you want that build I’d just go Cleric > Priest 2 > Paladin, this way you will get it when it is rank appropriate. This is a CBT however, so feel free to test and experiment as you want.
Another thing to mention is that there aren’t a terrible amount of INT scalings for this route like the good old full support RO Priest. You will instead benefit mostly from SPR, and INT can even be left at the minimums.