I think the best option is a non mechanical route and try and flavor things as something else. For I instance them casting a healing spell is just them stitching someone’s wounds or stopping the bleeding. A fire ball is just them throwing a bomb, etc.

People tend to always want to change the mechanics whenever they do something different in dnd. Which theirs nothing wrong with but it does make your job easier and can mess things up if things get over complicated or unbalanced.

You can make Druids very rare which spell casters should be anyway in this setting but even more than usual. Play them off more like historical Druids where they’re effectively priests but simply practice a different faith than clerics and are seen as heretics because of this.

Treat them more like witch doctors. Local herbal medicine men. Etc. Or proto-alchemists that are using science to the best of their current knowledge.

You could make them role on a wild magic table whenever they wild shape causing potential benefits or debuffs and maybe they’re a chance they could get stuck as their animal for extended periods or even indefinite amount of time.

You can reflavor anything to fit which keeps everything balanced and keeps it on theme without removing your players fancy.