The KDE Connect I'm using is the small app that helps different devices communicate. I mostly use it to send URLs and files between computer and phone. As far as I know it cannot handle email.
I've played around with KMail/the KOrganizer suite as a potential Thunderbird replacement. I found it to be OK but not great, in the sense that if I had to I could live with it but there are several things about it I wish were different. At one point I switched from Thunderbird to Betterbird in the hopes that it'd provide some buffer against AI Mozilla is/will be putting into Thunderbird. Switching from Thunderbird to Betterbird was trivial.