Basically how about when buying infra you can like buy different quality infra.
Like level 1 infra is the same population and resistance metric as the current, level two can house more people and is more resistant to attacks, etc etc. Someone also suggested random events some point in this forum - maybe upgraded Infra is less potent to event damage.
To implement just make everybody's current infra level 1 and put forward the option of upgraded, ig. Maybe level two infra gives you like an extra improvement slot every (set) amount of infra or smth. Idk.
Hell, maybe the credit purchase option that goes to buying infra could buy like a level of infra that can ONLY be bought with a credit. To make it not PtW you could just make that upgrade very slight but whatever.