Upgrading your organization from current versions to Exchange Server SE

Mr_Tomasz, ME3 (or more specifically Microsoft 365 E3) is a Plan, not an add-on.  There are add-ons for ME3 (e.g., Identity & Threat Protection, Information Protection & Compliance, Microsoft 365 E5 Compliance, and Microsoft 365 E5 Insider Risk), but ME3 is not an add-on.  If you have ME3 today, and you keep your ME3 subscription active, then do you get unlimited Office Server licenses for your organization, but only users or devices with ME3 licenses can access those servers.  But yes, a benefit of ME3/ME5 is the Extended Use Rights for Office servers (see Microsoft Product Terms for an example).

CU15 for Exchange Server 2019, due later this year, adds support for Windows Server 2025.  So, Exchange Server SE will support Windows Server 2025 from day 1.