There's a few associated problems to solve. not impossible, just work to be done. Here's some off the top of my head:
When you go to that site and enter your master key to decrypt all the passwords, how do you guarantee that the script served by the site is the version of the script you expect, and not some other script that quietly posts your master key to a remote server? TLS + your own custom certificate + serving all resources from a domain that only has content you control ( not GitHub pages domain filled with content from arbitrary people) would help for a start. Maybe also https://developer.mozilla.org/en-US/docs/Web/Security/Subres...
Need to be able to have reasonable confidence that your deploy pipeline and your repo holding the application code cannot be tampered with. How do you do that?
If you outsource storage of your encrypted passwords, how do you know your vendor still wants to provide service to you next week? Why should they still want your business? What do you do if they lose your data and cannot restore from backup? Might be worth spending a little money here for privacy & alignment of interests, or hosting it yourself.
How do you know you got the crypto right?
For anyone who isn't you, how do they know you got the crypto right?