Ahh, because you went with the HEN solution I thought you were just keeping your discs safe, then downloading the games through internet. So I thought downloading PKG instead of ISO in this case would be the solution.
How big is your disk? isn't there any chance you can get at least 1TB HDD? I've seen them around internet second hand for 10-15€. Of course, the ideal scenary would be to get an SSD, but 1TB SSD even second hand can cost around 30-40€, if you're very tight on budget and don't mind HDD slowness, then size would be the priority.