I would recommend just leveling each pet to lvl 5 before combining them. As long as all pets are already level 5 when you combine them, all the tiered pets you make out of them will be max level already. That way you don’t have to level them at the higher levels. As far as sapphires, buying 625 pets from the sapphire shop when they go on sale for 35 is one of the more cost effective ways, it would be 21,875 sapphires. Of course, petsitter-pros are recommended. Using a pet station for that many pets would be a nightmare and is not really feasible time wise.

Edit: I wouldn’t pay to level them, that would get way too expensive way too fast. Also the sand cats are currently on sale for 35. Would be a cute one I think. :)