I would pick the 2A4 but
you probably dont want to hear this, but maybe you should grind with talismans. It allows you to experience the entire techtree step by step and you can choose to talisman the tanks you like, so you dont risk spending money on a tank that you dont enjoy.
Some benefits:
+you actually experience the techtree with all its gems
+you dont have to play the same tank for hundreds of battles
+you are not forced to play a certain BR
+you will have a line-up
+you level up your crews in an more beginner friendly enviroment
+unlocking the techtree leopard2a4 feels much better. It gives you a goal that makes the grind more bearable.
Otherwise you grind in a Leopard for another Leopard. And you dont run in to the situation where you have to decide if you want to grind the 2A7 with the 2A5 or stick with the premium for a more efficient grind.
After multiple maxed out nations and techtrees I can tell you that the road to the next tanks is actually more fun than the tank you unlock itself. Maybe its just me, but usually once I reach the end of an techtree I dont actually play it alot, usually I begin a new techtree.
So imo talismans are the best way to grind fast and have alot of fun.
I know it can be more expensive depending if you have good timing with 50% off sales and how many talismans you decide to buy, but I would value fun and time over money, especially when you are in a life situation where you have enough extra money.
sry for the long text