Elderspring and Jadebark are both solid. I think Springbloom's Bloom effect might be a little overpowered now, since it gets you a Sleepy Dragon for (6) if you have the right hand size. If you're operating under a "if you hit the Bloom you get a (1) cost effect bump" then maybe change it to gaining +4 Health, which would make it a 4/10 Taunt for (6) and put it in line with Hungry Ettin.
As to the legendary minion, first, gonna say that we've definitely been drinking from the same inspiration pool since this was going to be my Legendary Boomsday scientist:
While I obviously think the start of game effects are great design, I also think it's something you'd need to seed and support more throughout the class cards. As is, of the minions and weapons in your class only Amethyst Staff cares about how much Spell Damage you have (as opposed to the various cards that care if you have a Spell Damage minion, which your hero having it wouldn't help), and only Sharpetal Dagger, Living Rose, and Regal Rosebeast add spells to your hand (and the first two are more Razorpetals). I understand the Baku connection would push this to a very "ALL the Razorpetals" style of deck, but contrast with the various minion generating spells that Hunter and Mage get in return for running no minions.
Of the other options, I like the Shudderwock-alike (although you'd want to make sure that Riverbound Cleaver is changed to just "give this" instead of "give this weapon" so it makes sense for Gnarlroot) since as you mention it can fit into any style of Bloom deck. #1 does have the benefit of being sneakily and flavorfully splashy, which is always a good aspect of class legendaries.