Good idea to just use imap clients using auth2, eg Thunderbird.
The best solution would be jmap but it seems dead, most implementations are dead and not compatible with the last version.
Do you use them as your domain registrar as well? They say to have a custom domain to transfer the domain to them, seems a bit extreme but I wouldn't mind consolidating my services.
I use Evolution with custom flags and virtual folders + imapfilter to automatically add custom flags. It works great.