What a piece of crap of webmail that is.
I do have a lot of complaints related to mail but if NC is any kind of useful replacement for MS365 / Google Workplace a decently working webmail is the bare minimum. RoundCube is WAY better than what NC is currently offering.
As said, they’ve separate repositories, annoying messages asking you for a license all the time etc. At some point you’ll find out that their solution doesn’t offer anything particular of value that you can’t get with other less company dependent solutions like I described before. You may explore the LXD native GUI… or heck even Cockpit or Webmin might be decent options.
Roundcube always! I tried others and always went back to Roundcube. If you install it properly (using composer for everything) it won’t break on updates.