but I would really enjoy having a Sunfire "beam" spell (i.e. holding left trigger to fire a continuous Sun Fire attack until magicka is depleted). I just would find that Sun Fire at its base is rather weak until combined with certain restoration perks. It's really cool to have an offensive restoration spell. Does anyone agree?

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I think that this is a spell more for RP, people who wish to specialise in the restoration and not in Destruction, much like the spectral arrow from conjuration. What you want is the novice and master destruction ( lightning ).

I'm not sure why he's recommending lightning as a suitable replacement for sun damage.

I do agree OP, that sunfire is pretty awesome. And weaponized restoration is nice. A concentration spell would be quite cool as well.
But you may be pleased to know that there is a more powerful version of sunfire, called vampire's bane. And a cloak spell version, called Stendarr's aura. And they are both quite good.