Mullvad browser, while excellent, is not really meant to be used as your default daily driver. It really is meant to be used for select use cases unless you absolutely have a need to use it as it everyday.
The set up you’ve explained works but I would still try to avoid bundling a non FOSS app with 1Password unless your threat model allows it.
I personally would simply harden FF as much as possible and use it as best as you can for whatever your needs and use cases are. And keep Mullvad browser for select use cases only.