Packaged RTU heat reclaim efficiency appears limited to 30%

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You aren't missing anything as far as modeling hot gas reheat with the Coil:Heating:Desuperheater object - the 30% heat reclaim recovery efficiency is a hard-coded limit.

However, it appears that EnergyPlus v9.3.0 now includes a different method of modeling condenser hot gas reheat in the Coil:Cooling:DX object (relevant PR here). I haven't tried this out yet, but based on this description it appears promising:

The goal of this new feature is to provide a DX cooling coil capability to meet all possible demand with different sensible and latent loads by using different mode operation in a single cooling coil. In other words, the coil itself can meet sensible and latent loads simultaneously without any additional help.