Personally, I would use Simple File Manager.
Simple File Manager (A simple file manager for browsing and editing files and directories.) -
https://f-droid.org/app/com.simplemobiletools.filemanager.pro
As for that developer setting, I have played with it before.
Suffice it to say that if you are camping (or similar) and interested in extreme battery conservation then it may be useful.
Other than that, the Android OS in modern phones does pretty well at not running a bunch of extra stuff that it doesn't need.
Therefore, unless you are in a situation where charging the phone is going to be an issue, I would leave that setting in the standard mode.
Keep in mind that I do more with my devices than most folks do, and yet I use pretty low end devices.
Rarely do I have memory issues, even on my 1GB device.
Androids have a 'recent apps' button on the navigation bar, that allows you to close apps that you have used recently.
For most purposes, this is adequate.
If you are still experiencing such symptoms such as auto restarts, freezing, app crashes, etc., then you should look at how much memory you have open on the device.
A general rule of thumb is to leave at least as much memory unused as the device has in RAM.
So leave 1GB unused if the phone has 1GB RAM.
Even better if you can leave 150%-200%.
This is just what works for me.
If you really want to get ahold of your running apps, I use Greenify.
https://greenify.en.uptodown.com/android
If you want to also try your hand at every single app on your device- including system apps- there is one called SuperFreezZ.
SuperFreezZ App stopper (Entirely freeze all background activities of apps.) -
https://f-droid.org/app/superfreeze.tool.android
I include it as a curiosity for you to play with.
I cannot say that I like it much, in fact I like Greenify much more.
You will probably find that any system apps that you close will soon start up again.
Sure, there are a few that won't, especially for two of my devices that I only use on Wi-Fi.
Anyway, I would get whatever you can off of that old SD card and replace it with a SansDisk brand card.
I have had nothing but good luck with them, and nothing but bad luck with the other brands of cards that I tried.