I apologize in advance as you probably don't want to here this.
:) apology accepted!
I have a replicated system and tried to use NFS to a file share server with dedicated gigabit links etc and my second replicated system.
I have 300+ accounts and many have 20+ gig of data over 600+ folders, your setup seems larger.
I spent 2 months trying to make this work reliably with nothing working out.
that being said (and this IS NOT a dovecot thing) NFS simply will not work reliably especially in the environment that you seem to be using
I went to local SDRAM drives on the second server and have had zero issues since.
Even a bit bigger setup would run ok on ceph. I have more or less default setup but have split up mailboxes on ssd and hdd, indexes on ssd. Although it is much slower than native performance. You have complete redundancy.