Great news, there is an official commitment to add support for certain ARM architectures by Checkmk: Checkmk – our road to ARM support. This could mean, that at some point the Raspberry Pi 5 and subsequent models could use native Checkmk packages. In the meantime, you can try Zabbix, it might even be the better choice anyway.
Unfortunately, due to native ARM support on the horizon we have decided to retire this project (thank you @martux69 for all your hard work supporting arm64 over all the years). This decision was based on the fact that keeping up with upstream changes has recently become increasingly time-consuming, additionally other (personal) things have become more important, so that there is no longer enough time.
There is a fork of the project that is being maintained on a "best effort" basis: https://github.com/FloTheSysadmin/check-mk-arm.
Packages for current Checkmk versions (2.3, 2.4 and 2.5; arm64; Debian 12/13 and Ubuntu 24.04) are available here: https://github.com/sergiodadi/check-mk-raspberry.