Are you referring to cache, storage, or both? Did tapping “clear cache” in your phone settings work?
There’s an existing feature request for adding a “clear cache” option within the app itself, though you should be able to go to settings>Apps>iNaturalist>Storage and cache to clear the cache, storage, or both.
→Looking through past posts, there are already measures in place to limit and clear the amount of data being stored on the device, including removing full-size photos.
(As far as your long password, password manager apps/software like KeePass definitely come in handy!)