I purposely went with a third-party password manager for this very reason. I wanted to be cross platform. It's the same reason I wouldn't use Google's chrome password management. I wanted the flexibility of using my passwords on any device/browser/OS. Not have to be concerned with porting my passwords as I have almost 400.
Apple allowed 3rd party password managers to integrate in iOS so I wasn't losing any functionality going that route.
I also realized when I considered iCloud Keychain for passwords that they might have a security model that would make it harder to simply export them all at once. Me personally, I'm all for more security of passwords which might give less flexibility which I believe is why Apple requires an Apple device to obtain them.
My passwords are one of the most important thing of my digital life, it just stood to reason that using iCloud Keychain or Google to do that was counter-intuitive to having the most flexibility on where to use them.
I use both 3rd party and keychain. It’s pretty easy to keep them in sync as keychain automatically asks if you want to store or update a password when a new one is added/changed.