##Are you sure this is a druid?... While I don't think it's unbalanced, I think it's too divergent from what a druid is supposed to be about - nature. Furthermore, the sorcerer aspects don't really match with the idea of a "scientist". That strikes me as something more wizard-like, very Dr. Jekyll and Mr. Hyde, rather than anything druidic.

I’m not a fan of inventing a whole new class or subclass.

Making it a subclass rather than messing with the base mechanics of an existing class is actually a better, perhaps even simpler option. If I were to create such a character, I'd probably look at porting some of the wildshape mechanics (including your monstrosity choice) over to a Wizard subclass.

##If it's gotta be a druid... If you're dead set on Druid, I would still do it as a subclass. Don't mess with the spell casting, that's your biggest potential imbalance point. Sorcerer spells tend to be direct damage, while Druid spells tend to be lower damage but more subtle, or have support effects.

Go ahead and give them Alchemist Tools proficiency for free when they pick up the subclass; one bonus tool isn't going to matter much.

However, monstrosities tend to be have strange abilities. One of the balance considerations to take into account is how often they'll get to use them. Many of the creature abilities become more powerful when they're regularly available - they're designed with creatures that end the encounter dead in mind. With that in mind, I'd base your Monstronity Shape ability on the Circle of the Moon, with a few changes.

• Don't take away the basic druid's beast abilities; there aren't many good monstrosity choices at lower CRs.
• Combat Wild Shape could stay the same, but due to the special effects available to monstrosities, I would probably drop the bonus action transformation part. A slight delay in the power up phase will balance out those special abilities to some degree. Keep the healing, perhaps combine it with giving them alchemist proficiency and call it Alchemical Enhancement or something to that effect.
• Circle Forms stays the same, except that it adds higher CR monstrosities not higher CR beasts. The character will retain the utility function of the normal, lower CR beasts, but ends up with a narrower range of monstrosity forms. Call it Monstrosity Forms instead.
• Primal Strike is fine, maybe call it Alchemical Strike for flavor's sake.
• Elemental Wild Shape is a bit problematic for the theme. My initial thought was to add a few aberration forms, for extra weirdness, but there weren't many good choices to match the CR 5 of the elemental forms. In the absence of anything better, I'd just leave it as-is.
• Thousand Forms doesn't need any changes, other than perhaps calling it Alchemical Adaptation or something like that.

Are you sure this is a druid?...

While I don't think it's unbalanced, I think it's too divergent from what a druid is supposed to be about - nature. Furthermore, the sorcerer aspects don't really match with the idea of a "scientist". That strikes me as something more wizard-like, very Dr. Jekyll and Mr. Hyde, rather than anything druidic.

I’m not a fan of inventing a whole new class or subclass.

Making it a subclass rather than messing with the base mechanics of an existing class is actually a better, perhaps even simpler option. If I were to create such a character, I'd probably look at porting some of the wildshape mechanics (including your monstrosity choice) over to a Wizard subclass.

If it's gotta be a druid...

If you're dead set on Druid, I would still do it as a subclass. Don't mess with the spell casting, that's your biggest potential imbalance point. Sorcerer spells tend to be direct damage, while Druid spells tend to be lower damage but more subtle, or have support effects.

Go ahead and give them Alchemist Tools proficiency for free when they pick up the subclass; one bonus tool isn't going to matter much.

However, monstrosities tend to be have strange abilities. One of the balance considerations to take into account is how often they'll get to use them. Many of the creature abilities become more powerful when they're regularly available - they're designed with creatures that end the encounter dead in mind. With that in mind, I'd base your Monstronity Shape ability on the Circle of the Moon, with a few changes.

• Don't take away the basic druid's beast abilities; there aren't many good monstrosity choices at lower CRs.
• Combat Wild Shape could stay the same, but due to the special effects available to monstrosities, I would probably drop the bonus action transformation part. A slight delay in the power up phase will balance out those special abilities to some degree. Keep the healing, perhaps combine it with giving them alchemist proficiency and call it Alchemical Enhancement or something to that effect.
• Circle Forms stays the same, except that it adds higher CR monstrosities not higher CR beasts. The character will retain the utility function of the normal, lower CR beasts, but ends up with a narrower range of monstrosity forms. Call it Monstrosity Forms instead.
• Primal Strike is fine, maybe call it Alchemical Strike for flavor's sake.
• Elemental Wild Shape is a bit problematic for the theme. My initial thought was to add a few aberration forms, for extra weirdness, but there weren't many good choices to match the CR 5 of the elemental forms. In the absence of anything better, I'd just leave it as-is.
• Thousand Forms doesn't need any changes, other than perhaps calling it Alchemical Adaptation or something like that.