Neville:
Acknowledged, I go with the Mozilla flow for accuracy and consistency. However, exporting calendar and contacts frequently to my local external drive creates a backup that I can use outside the Mozilla world when a distro comes equipped with Evolution or a different PIM. I can also export my Firefox bookmarks and use them in a different browser, although Firefox is my browser of choice for security reasons.
Following the principles of Linux and Unix can be a desirable intent, but practically less than useful when there is a secure and more simple way to get the job done. Adherence to FOSS principles to the exclusion of using Occam’s Razor seems to me to be a rabbit hole practice. I try to avoid those by making secure decisions and avoiding Google and Chrome.
BTW, Protonmail is developing into a very useful provider of practical, secure solutions.