I think a key point here is making this 'pluggable'
ie like a contrib - it can be installed or removed and swapped without interfering with things?
If we use Roundcube (and seeing the potential tie in with Nextcloud it seems fairly logical) is it easy enough to use Sogo or a.n.other easily?
Snappy is interesting but seems a bit of a one man project? Seems to have various hooks to Nextcloud.
https://github.com/the-djmaze/snappymailSimple, modern, lightweight & fast web-based email client.
The drastically upgraded & secured fork of RainLoop Webmail Community edition.
We thank the RainLoop Team for making a great PHP 5 product that was good in the past.
Up to date system requirements, snappy performance, simple installation and upgrade, no database required - all these make SnappyMail a good choice.
https://apps.nextcloud.com/apps/snappymail