Forgot to say that yes, CalyxOS does have microG, though you don’t need to log into Google to download apps from Aurora. Login is only required for apps from Google (like maps, gmail etc).
I also got the Fairphone 5 because of the used price! Mine was 300€ with a slightly burned in screen (it was used as a store display model), though I only notice it when on a completely white screen and looking for it.
I also use mailbox.org, specifically the 1€/month tier. I’d recommend it.
And if you want to be 100% sure that Alpaca doesn’t send any info anywhere, you can restrict it’s network access in Flatseal as it’s a flatpak.