A truly secure email server can’t access your conversations, can’t profile you, won’t show you targeted ads, and won’t log any data or metadata. You can set up your own mail server using Mailcow, which has a nice modern web interface, and a calendar function.
Yes hosting it yourself will be a bit more initial setup effort including creating a hosting environment (home IP addresses are often blocked by commercial mail providers - it does provide DKIM and ARC support though), and domain name, and the Mailcow installation. The management and interface though are a lot more functional and modern looking than you get with typical self-hosting solutions such as Roundcube, Rainloop, etc.
From what I see though, the security is centred around login access and hosting privacy, as I don't see anything relating to encrypted mail. The problem with encrypted mail though is all your recipients also need to be using the same encryption, otherwise they cannot read it. Truly encrypted mail, with something like PGP encryption, is available with Thunderbird mail as even with 3rd party add-ons in GMail.
See How to Set Up Your Own Secure Email Server
You can protect yourself from potential cyberattacks and take more control over your email account.