This works great:
https://www.skraper.net/
One thing to check in the options is how it checks the names of the game, by CRC value then name, backwards or one or the other. Personally I choose to check by name. For example, I use my own PS1 ISOs converted to PBP so when it checks CRC values it only downloaded Point Blank game data (which I didn't even have, possibly what pbp converters use). So since I have everything named neatly, when I chose to use the file names instead, it downloaded everything really nicely.
Also you can add videos or logos to download as well (by default for recalbox it only downloads game cover and screenshot I think)