Over 500 maps, wow! I never expected anyone to keep so many maps on their phone at the same time. I thought I have a lot of maps but the size of my collection is probably 100-150 and I only keep about a dozen or two local maps on my phone, and I organize the rest in Google Drive, and load them on my phone when I need them.
I will release the next version probably sometime in the summer (June/July). And I am planning on including a feature that will allow importing all the maps in a single directory, though due to the new limitations of Android OS that directory most likely needs to be somewhere under Downloads.
Regarding loading maps with spaces in the filename: Maps with spaces in the filename work fine because they are processed differently than the map images when maps are created. The failure to process spaces and most special characters is present only when creating new maps, and only applies to map images, not to map names.
Regarding maps appearing after deleting the app and installing it again: When you delete a map in 1.8.1, the map is only deleted from Custom Maps app's internal storage. And if you uninstall and reinstall the app, and you give the app storage access permission before launching it, it will check if the CustomMaps folder contains any maps, and try to import them all (only when the app is launched the first time). But if you remove or rename the CustomMaps folder, the maps are not going to come back by uninstalling and reinstalling the app. BTW, the next version of the app will not have this feature any more, since including the storage access permission is no longer allowed. The app already does not ask for the permission anymore.
But it just might be possible for you to import all your 500 maps into CustomMaps by placing them all into the CustomMaps folder, uninstalling and reinstalling the app, and checking that it has storage access permission before you launch it. And when you launch it and accept the license, there might be a long pause without any feedback while the app copies all of the 500 maps into the internal storage. I did not create a progress indicator display for the transfer because I didn't expect anyone to have so many maps on their phone that the transfer would take a significant amount of time. I assume that if any of the transfers fail, the process gets interrupted and the rest of the maps don't get transferred. If you wait patiently, the map list should get populated with all of the maps.
Once you have transferred the maps, I recommend copying the maps in the CustomMaps directory to a safe (online?) storage, and then clearing the CustomMaps directory, since the app will not use that directory again for map storage after that point.
Author of Custom Maps