Guardian without unlimited missiles is at least on par with Radiant, and if Radiant will be bumped to 60 DP, then so should it at the minimum. If Guardian keeps unlimited missiles, then it probably should be worth more, at least 75 DP (and more is not unreasonable). Ziggurat has unlimited motes (which might as well be missiles), but the blue ones the player can use do no damage. Guardian's unlimited missiles will cause damage.
Also, playable Guardian could share Ziggurat's other unique quirks, namely extreme CR deployment cost and/or distinct profile that auto-identifies your fleet (so no stealth ops for you).