You can use any IMAP/POP email client with G-Suite.  There are both open and closed source paid and free email clients out there.  This article may give you a high-level idea about the different alternatives: https://www.techradar.com/best/best-email-clients

Calendar is a slightly different animal.  To access Google calendar, whatever email client you use will need to have an API.  Thunderbird has extensions for that but flexibility of modulation also is the biggest shortcoming - Thunderbird has a history of updating the software ahead of the extensions being ready and that causes extensions to fail (at least for a time) until those extensions are subsequently updated (although may some actually be abandoned by developers).

I would also consider the availability of end-to-end encryption for emails to external recipients, not by PGP, which is too technical for customers who are novices to use, but through the provider's web-based portal.  This functionality is available natively in MS 365 but not G-Suite.  Encryro touts itself as a viable addin for encryption but I don't know about how secure the app is and you'd want to do a bit more research on that.

In summary, I don't believe there is a true contender to Outlook when it comes to functionality and calendar integration but there are others that come close except those would generally require a bit more work for configuration and maintenance.