I do the same: I save each new airport I bought / downloaded in Google maps (created a MSFS airports list). This helps a great deal in flight planning, because one quick look at the map gives me an instant idea which custom airport is within reach for my next flight.
Additionally, I keep an Excel list with columns for:
Continent
Country
Airport name
ICAO Code
Bought where?
Developer
Purchase date
Price
Remarks
I admit this might look like overkill, but it only takes a minute per purchase. The benefit is that I see immediately what I own, how much I spent, if it makes sense to re-purchase an airport from a different developer, etc.
And for those who are into pivots, you can keep analyzing till kingdom come…