The question is: How can I protect my credentials in a comfortable way?

These are my requirements:

Open source so it can be audited and its security is always testable
Available for Linux, Windows, Mac, Android and IOS
2FA using Yubikey

I am also available to pay for a good product.

Pros:

     Password Safe - Designed by a renowned security technologist Bruce Schneier
- Simple and light app. You can save just your credentials, URLs and a note, nothing else
- Drag and drop username and password to fill in forms. No autotype means more control and less risks on scam sites
- Master password is saved in an encrypted form in memory. It cannot be dumped
  Masterpassword - Stateless password manager. No files to sync. Every time passwords are recalculated using your master password, username and URL
  KeepassXC - Famous and adopted even from european governments
- You can use key file to unlock the db
- Fast application
  Bitwarden - Clean interface
- All is synced in Bitwarden’s cloud. You can also setup your own server version
- Automatic check if you password has been pawned

Cons:

     Password Safe - A little bit outdated app
- Few options to import password from other password manager but KeePass
  Masterpassword - If your master password is compromized and you have to change it, you must change all your password in all your sites because they are recalculated with the new master password
- No 2FA
- You have to remember exactly the URL you used and the version of the password if you changed everytime
KeepassXC - Many conflicts if you have to sync in a multi device architecture
- Browser extention sometimes fails to autocomplete the form
- Browser extention sometimes fails to update/create your credentials

Bitwarden - Browser extention and app are really slow
- Not easy to delete items
- If you are behind a proxy you may be not able to update youe repository

The answer is:

Password Safe - https://pwsafe.org/ designed by Bruce Schneier

and

KeepassXC - https://keepassxc.org/