That's a good principle in theory, but how do you know that using any 3rd party password manager is any better? There has been plenty of security problems with

That's a good principle in theory, but how do you know that using any 3rd party password manager is any better? There has been plenty of security problems with all of the major providers as far as I'm concerned. At least with all these DIY password managers which are open-source you can see all the code, which is not true for any 3rd party custom made solutions. Also, one of the reasons why I went with libsodium as an encryption solution is the exact same reason - I'm not a cryptograper and libsodium offers an easy and fool-proof API to not mess things up accidentally. This is not true for many other (popular) cryptography solutions.