Is that solely by running away and not provoking opportunity attacks? Because otherwise I don't see how this lets you keep up a concentration spell. And a form whose main use is "I got hurt, but I kept my spell up, so I need to run away, so I'm going to use my action to transform" seems like it has to hit a lot of gates to get used.
How is this better before a combat than casting a battlefield control spell? And if you are planning on leaving it during combat, why go into it in the first place? Is the sole plan here for stealth and ambush tactics? Because... firstly, Pass without Trace, and secondly, you would rarely want to initiate combat from stealth unless the rest of the party was near.
Sure, for a stealth situation, this can be good, but I'm not sure this will see much use. I mean, attacking from stealth with a d8 isn't great, and 1d4 + poison condition could be good, but I'm not sure how it is better than hitting a group of enemies with a 1st turn entangle.
This is the main thing I want the druid to do, but I don't think 1 hp and no ability to attack is quite fun. I don't want them to have a lot of damage, but being able to do some damage means they can pull some tricks.
See, this is where my experience just runs counter to your design. As a druid, I never wanted to conserve spell slots. I had too many spell slots as it was. I can't imagine the scenario. Also, again, this takes your full action. You are spending an entire turn doing nothing but assuming this form. Mid-combat? The design of many classes and many abilities over 5e have shown us that using an action for a self-buff is undesirable by many many players. You need to be able to take an action that affects the battle now, not next turn.
Also, one of the things I honestly do love about the current One DnD Druid is that they can go large from 1st level. This allows them to do things like turn in an ox and pull a cart out of the mud, or other utility options that are just fun or travel related. I don't want to wait for level 5 for that, if I don't have to,
If you want control... you are a druid. Battlefield control is the only thing you are really good at as a caster. And sure, grabbing a huge monster in a grapple is fun, but... is that worth spending an entire action to set up? I don't think it is.
And this is the thing I keep butting up against with the idea of using wildshape effectively in combat. Either you give up casting your spells before the fight, you skip your first turn, or you skip your second turn. And nothing that would be balanced to take the form of as a bonus action, is worth skipping your turn to accomplish.