What is the "predecessor of this site"? GPSFileDepot has been around for quite some time.... I have been posting maps here since 2008, and it was not new then.

But anyway, it's meaningless to talk about the "maps from this site" as though they had something in common. They do not. This site is just a collection of maps by many different people. There are no standards for how they are made and there can be big differences in the approach and method. of the authors. In other words, it's like talking about the "books from the library", as though they all had something in common.
Most of the maps here should work on a Nuvi. If you are having problems with all of them, then you must be doing something wrong (or you're incredibly unlucky in choosing maps that have problems).
I spent a fair effort to make my own maps look good on a Nuvi. They may not cover an area you want, but they definitely work on the Nuvi - try this as a test:
https://www.gpsfiledepot.com/maps/view/785/Are you saying that you have an SD card where your old maps work but new ones installed at the same time don't? That would be odd. How many maps total do you have on the card? Where are they installed? Newer Nuvis like yours expect the map to be installed in a folder named Map. If the card is in a computer slot or USB reader (instead of inserted in the GPS) when you install the map, then Garmin's software may not do this correctly.
Also, with Mapsource, when you install a new map, it will erase an old map on the card because it always uses the filename gmapsupp.img. MapInstall shouldn't do that. You can also manually re-name gmapsupp.img to something else, like topo.img after installing.
If you don't format the card as FAT-32, then it will not work in the GPS. How big is the card? Cards larger than 32gb will not work properly.
Also, are you sure you have the newest version of Basecamp? Because the version before that was broken an could NOT install most of the maps from this site.
Just some ideas. No idea what's wrong, but almost all the maps here should work on your GPS.