Lease or Purchase by Manufacturer
Instead of leasing/purchasing by specific types of aircrafts, have an option to purchase from the manufacturer for all types. In the real world, airlines purchase aircrafts not by type but by manufacturer. It allows them to purchase multiple families and types in one purchase, increasing their likelihood of getting a discount. In the game, the airline can only purchase one aircraft type at a time. For example, if an airline wants to buy 4 A220-100, 4 A220-300, 4 A319, 4 A320, and 4 A321, they would get no discount. However, if they purchased all of them together from Airbus, there would be a total of 20 aircrafts which would give them a 30% discount for the total purchase.