What e-mail do you use?

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.

For us, we use Protonmail with our own custom domain primarily for security considerations.

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@ljsmith2 wrote:

Is MS 365 a web based email?

Is there another version of Outlook for desktop?

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Microsoft 365 is a subscription based program (monthly or annual fee) that they update it to the newest version every 2 or 3 years.  Most versions are BOTH cloud based or desktop based (but some of the subscriptions are only cloud based).

I'm old fashioned, so I hate that.  I want to OWN it (not pay an annual subscription fee), I want it on my own computer (not the cloud) and I DON'T want them to keep completely changing my programs every few years so I have to relearn where everything is.  

So personally, if I were to buy Microsoft Office (which includes Outlook), I would buy the non-subscription Desktop version (NOT 365, the current version is called "Office 2019").  See link below for an example (I'm not endorsing whoever the seller is on the link below)

https://www.amazon.com/Microsoft-Office-Business-Windows-Download/dp/B07H4X8QF4

HOWEVER, if you purchase the non-365 program, it is only valid on ONE computer.  If you use Microsoft 365, that is usually valid up to 5 computers.  So if you want it on multiple computers, then 365 may be a better deal.

https://www.microsoft.com/en-us/microsoft-365/get-started-with-office-2019#office-ContentAreaHeadingTemplate-eri5pno

https://www.microsoft.com/en-us/microsoft-365/business/compare-all-microsoft-365-business-products