After many years of support, Microsoft no longer produce a direct successor to the Small Business Server and Windows Server Essentials whcich the best out-of-the-box small business support and configuration. There is still a 2019 Essentials Server which has a very attractive licensing package for organisations with fewer than 25 employees which provides a full range of server functionality and can be configured to suit your needs. - Microsoft Windows Server 2019 Essentials.
The effective replacment for the above servers is the Cloud based collaberation platform from Microsoft - inclduding desktop versions of the Microsoft Office suite - Microsoft 365 (formerly Office 365 business) - it includes Enterprise class email and the much advertised Microsoft Teams
Our recommended antivirus for small offices is from Trend Micro, who provide both on-premis server support and an excellent hosted solution - the-it-service is a Trend partner.
Check the effectivness of your firewall using Shields Up! from Gibson Research.