HA between different appliances

This solution has a lot of information about your question:

https://support.f5.com/csp/article/K8665

You need to check what modules you will use, as APM for example as per solution will not work.

The other problem is connection mirroring, as there is a TMM connection from one unit to the other, and the number of TMM is directly related to the number of cores, so you are likely to have problems with connection mirroring.

There is also a major rule that you not following, is that the standby unit should be able to handle the same traffic as the active unit (and vice versa), otherwise you will have surprises when the active device failover.