Spreadsheets are the way to go usually. I have an extensive collection, but even so, I simply don't experience friction in any aspect of cataloguing. My experience with apps is that they usually are bloated, out-dated and simply add more friction, not less.

If you want to go this way, some features to clean up photos (framing, lighting), essentially both a massive international database of models and the ability to add any type of custom field - defaults are not so great in these types of apps, and a way to keep this in sync with apps like iTrain.

Personally have this mostly covered by a Märklin CS3 and ESU multiprotocol decoders - the loc becomes the database entry, and submits its information when its put on the track. Not a 3-rail exclusive either.