I've been thinking about Garth for the past few days so I may as well post some thoughts.
I do think he's potentially pretty dangerous - infinite loops isn't a HUGE risk I don't think, since he generally needs haste enabler + something cheap to blink him...I mean there are definitely ways to do it, but I don't think they're easy enough to be the primary issue. I think the bigger issue is simply that having repeatable regrowth with many control spells can become real miserable real quick. Recur cyc rift every turn, recur
Duneblast every turn, even recurring counterspells gets pretty unpleasant in a lower-powered meta. And while it's not deterministic, his braingeyser is a pretty potent draw engine that could easily hit similar effects and ways to blink him, so even with single-shot blink he'll probably be able to keep going and going and going.
To avoid this, I think the only good options are to eschew almost any form of blink or recursion, or to cut anything that becomes problematic when cast every turn.
Personally I like my control decks so I'm leaning hard on option A.
The question for me is, what about a single, one-use recursion/blink spell? That would give the ability to blink him over and over, but you'd have to spend the regrowth mode to do it. On the other hand, it does enable repeated braingeysers. With a haste enabler, you could basically just spend 3 mana to reset him, and then dump the rest into braingeyser + control spells every turn. Not deterministic but definitely nasty.
Maybe a singleton recursion spell would be better - it's really slow since it costs the spell plus his mana cost (or the usually-hefty cost of reanimation), plus he'd have to die naturally if you don't include any sac outlets. And it'd also provide some protection, though if it's a singleton card in the deck and you leave him in the bin, you might end up getting him stuck if the spell is countered - but that could be remedied with grave hate to pull him back into the CZ without increasing the number of cards that reset him without cost. Still could be obnoxious with enough mana I suppose, but so is everything really. But maybe it's best to just go completely bare, and only reset him by letting him die and pay tax.
I think I'll be happy when I find myself consistently running through 4-6 spells before resetting him. If the right lines involve just repeatedly doing the same 1-2 options over and over, that seems like a problem.