Learn Turkish: Keşke bu kadar savurgan bir adam olmasaydın. - If only you weren't such an extravagant man.

It comes from the verb savurmak, which means "to scatter," "to toss," or "to blow away" (like wind scattering leaves). The suffix -gan attaches to the verb root to describe someone who has a strong habit or tendency of doing that action. In this case, someone who "scatters" their money or resources is "extravagant" or "wasteful."

Why does the translation say "weren't" when olmasaydın contains the past tense suffix -ydı?

The combination of the conditional -sa and the past -ydı (-saydı) is used for unreal or contrary-to-fact situations. When used with keşke (if only / I wish), it can express a regret about the past ("If only you hadn't been") OR an impossible/unlikely wish about the present ("If only you weren't"). Here, it's a complaint about the man's current, ongoing nature.

Could I say bu kadar bir savurgan adam instead of savurgan bir adam?

No, savurgan bir adam is the correct order. As a quick refresher: in Turkish, when a noun is modified by both a descriptive adjective and the indefinite article bir, the adjective almost always comes first (Adjective + bir + Noun). Placing bir before the adjective usually sounds unnatural.

What is the exact role of bu kadar in this sentence?