Openmediavault + mergerfs + snapraid is very similar to unraid storage in that you can add different disk sizes just like unraid. Admittedly it's not as 'plug and play' as unraid, but it's free, so can't really complain. Disk speeds using this config are also much faster if that matters.
I have considered truneas for if/when I need to rebuild, but this works for my jellfyin/arr stack needs.
I just did this last week. I originally tried stalwart as that seemed to be a pretty small footprint (1 docker container) and had built-in PGP support. But I couldn't get the built in acme bot to work with my dns provider, maybe you'll have better luck.
I then tried mailcow and had e-mail flowing in under an hour. Of note, there are a lot of containers if you go the docker route, some may not be necessary if you just want email. I have so far left the default containers running as I don't really care about the extra containers, it's all running on a vm w 5 GB of RAM. I'm not concerned about PGP at this point as the server hard disk is encrypted via LUKs.
I can't speak to your first 2 questions, dovecot comes with mailcow, but I didn't have to set any specific settings for it.
I port forward directly to the vm, but it's on it's own isolated VLAN. use 993 for imap instead of 143, it requires SSL if using a proper mail server.
I use thunderbird for desktop and fairmail for android. mailcow dockerized does come with it's own webclient SoGo, which didn't seem too bad if that's what you prefer.
I hope this helps!