1. There is no better open-source alternative for Windows for mail/calendar/contacts client.
betterbird has "better" in the name
because barely anybody is making desktop mail/calendar/contacts clients anymore. There is very little development in that market as most people have moved to web interfaces
And even Outlook is a web app these days, with "performance" to match.
There really isn't even if you don't need all the bells and whistles. I want my email client to be as simple and minimal as possible and Thunderbird seemed like the last candidate for this. Surprisingly it's the only one I could theme and strip down enough to meet my need.
Plain and simple is why I like macMail but it's a bit too simple and really mostly useful if you use iCloud email primarily.
Honest question, what do you even need a native client for? What can you do on Thunderbird that you can't on a browser?
Store e-mails offline.
If you use multiple email accounts on different platforms (private providers), client is only way how to manage it effectively. Same for calendars and contacts. You probably wont use web browser for edit contact on phone as well.