Is there some kind of ROM download and management software available? I don’t want to have to use my computer every time I want another game.
You should be able to use your phone to transfer if you have an android
you may also use network to mount a NAS or your computer hard drive to the gameshell using fuse
other possibility is to use an on the go usb wire to plug by example an usb key
Is there no way to navigate something like [Link Removed]
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Welcome to the forums! 
One thing you can do is make your own warehouse/repository, and have individual scripts linking to a private drop box where you may keep your roms.
It would be tedious, but I’m sure that we could have a script made to generate it ourselves.
Alternatively, if you can find a site that hosts the roms without any timers, bot checks, or any other things requiring human intervention, you could probably run a wget command, having each rom downloaded to the correct system’s directory. Again, it might be easier to just download all of the roms you’d want and upload them to your own fileserver, dropbox, google drive or whatever.
Either way, it would have to be one script per ROM request, so yeah. It’s a lot of work. For the time it takes to do, I would rather work my job for a couple of hours, earn money and just buy a 128GB SD micro card; slamming every possible rom I could want.
Besides larger disc images, ie playstation games, I’m having difficulty filling up the gameshell with complete rom sets for all of the included emulators. That’s including all languages and versions too. Dare I say it, even if you downloaded a complete ISO set for the PS1, you probably would still be able to fill a 128GB card. They’re pretty cheap, the equivalent in cost to 2 hours of work at the minimum wage in Australia.