I came up with this build for the [Pyrana Prime] as I toyed around with it a bit, it should give the maximum Sustained Damage.

For higher Burst damage I suggest switching out [Augur Pact] with [Pressurized Magazine].
It comes with the condition of reloading, but the Burst damage will jump from 225,336.8 to 274,801.0

You will need one less forma if you do not care about [Lethal Momentum] (which I only added to give it slightly better range.)
Alternatively, without spending a Forma on the Exilus Slot you can add a maxed out [Steady Hands] to reduce the recoil of this weapon.

Notes:
If you forgo the Exilus slot of the weapon, you will need only 4 forma.
I used Heat damage as it is currently (April 2020) the best single element from what I have read.
I did not use a Riven as the Riven disposition is quite low (0.6) and they cost incredibly much. But if you feel the need to add one, grab Critical Damage, Damage and Multishot. (Fire rate and Reload speed are also good to have on this gun.) And replace [Augur Pact] with the Riven, it has the least imPact of all the mods (besides the Exilus).

Criticism is welcome!